Is there a spell, ability or magic item that will let a PC identify who wrote a letter?

Question

In my campaign my players are working for a secretive organisation, and gets their orders through a magical scroll that lets them send messages back and forth. There are two copies of the scroll, and a copy will display whatever is written in the other.

They now suspect that someone is impersonating their contact, and want to try to locate either the other copy of the scroll, or somehow identify the person writing in the scroll.

Is there a spell, ability, or magic item that will let a PC do this?

So far I've identified Legend Lore, which could probably get them some information on the organisation they're working for, and Scrying, which (depending on how you rule) could let them target the sender. There is also the Retriever, even if it feels a little extreme.

Of these, Scrying is probably the most likely to work since it could conceivably let them target the individual writing in the scroll. Legend Lore might give them information they can use to get closer, but would not directly help them. The Retriever could help, but that would be a whole adventure in itself to get hold of one (which doesn't have to be a bad thing).

I am asking the question because I would like to know if there are more options presented in the official rules.

(I could just homebrew something, but it is interesting for me to know if such a thing already exists)

Additional info: The party is currently level 5, consists of a Conjuration Wizard, Moon Druid, Swashbuckler Rogue and Redemption Paladin. Solutions does not have to be available to these specific classes/levels, and answers do not need to consider the potential cost of materials/items/hired help.

Answer 1

Disclaimer

Some of the possible solutions I'm about to propose only work for certain methods the Imposter could be using for the deception; for example, they might have replaced the scroll your players are using with one under their control, or they might have stolen the original other scroll. There are too many possibilities to discuss them all, but I'll try to briefly mention what kinds of deceptions each potential solution will be effective against.

Visions of the Past

If the scroll you have was ever in the Imposter's possession, then the Knowledge Cleric's Visions of the Past feature should reliably help identify them. The relevant portions of the feature are:

Starting at 17th level, you can call up visions of the past that relate to an object you hold or your immediate surroundings. You spend at least 1 minute in meditation and prayer, then receive dreamlike, shadowy glimpses of recent events.

Object Reading. Holding an object as you meditate, you can see visions of the object’s previous owner. After meditating for 1 minute, you learn how the owner acquired and lost the object, as well as the most recent significant event involving the object and that owner.

Your party doesn't have a Cleric, let alone a 17th level Knowledge domain one, but perhaps they could go on a quest to find one and earn their services.

Locate Object

If the Imposter is nearby, such as in the same city, it may be possible to use Locate Object to find the other half of the pair of scrolls. This is a bit situational and requires a bit of DM ruling, but I would probably allow it. Here is the relevant text:

the spell can locate the nearest object of a particular kind, such as a certain kind of apparel, jewelry, furniture, tool, or weapon.

This spell can't locate an object if any thickness of lead, even a thin sheet, blocks a direct path between you and the object.

The DM would have to rule how specific a "particular kind" of object can be, but if you put your own scroll into a lead-lined box then locating another of that "kind" of object should be possible. Since it's a 2nd level spell with no costly components and it's available to your Paladin and Druid, and possibly your Wizard, you could cast it multiple times in a day to cover a larger city. It would be very impractical to search much further than that though.

Scrying

Scrying is very dependent on the DM ruling that "whoever is writing these messages" is sufficient to target someone, but if it's allowed then Scrying should work on a wide range of possible Imposter methods. There are some complications though. Scrying requires a focus costing 1000gp, which might be quite expensive for your party. It also allows for a saving throw, and since we're already stretching whether the target is even valid they would definitely get the bonus to their save from being not very familiar. Additionally, a successful save means you can't try again for 24 hours. Finally, your party won't gain access to the spell until level 9. All that together means it could take quite a while to actually get a result, though nothing about the spell's description indicates that the target will know about attempts whether successful or not so that might not be a problem.

Skill Checks

When players want to do something that doesn't necessary have official rules, it's appropriate to call for a skill or tool check (or technically an ability check that lets you add your proficiency bonus if you have proficiency in a particular skill or tool). Here are a few examples of checks that could be appropriate based on examples from the PHB section on ability checks and the Xanathar's Guide section on tool proficiencies with some emphasis added:

An Intelligence (Arcana) or Intelligence (Forger's Kit) check to determine that someone has messed with the magic of the item directly:

A forgery kit can be used in conjunction with the Arcana skill to determine if a magic item is real or fake.

An Intelligence (Investigation) or Intelligence (Forger's Kit) check to determine an Impostor has forged the scroll itself or is imitating your contact's handwriting:

Investigation. When you examine objects, proficiency with a forgery kit is useful for determining how an object was made and whether it is genuine.

A Wisdom (Insight) check to determine whether the contact has changed writing style subtly that could indicate they were replaced:

Your Wisdom (Insight) check decides whether you can determine the true intentions of a creature, such as when searching out a lie or predicting someone’s next move. Doing so involves gleaning clues from body language, speech habits, and changes in mannerisms.

Your players could also seek help from a Forger NPC if they aren't proficient in relevant tools and skills.

Divine Help

This option is the most dependent on DM rulings, and as a consequence has a lot of potential flexibility and could even be used for more indirect Imposter situations such as mind control or shapeshifters that even Scrying might miss because the writer doesn't look different. Some examples of divine help are the spells Divination and Commune, and the 10th level Cleric feature Divine Intervention. All of these have the potential to gain relevant information from a deity with the right wording, but it's up to the DM exactly what information the deity has and how they choose to share it.

None of your players naturally has access to these even at higher levels, so if you want these to be an option then you'll need to decide how they're accessible as well.

Answer 2

Mirror of the Past

from Tales from the Yawning Portal

The holder of this platinum hand mirror can learn something about the history of a specific object or creature by taking an action to gaze into the mirror and think of the target. Instead of the holder's reflection, the mirror presents scenes from the target's past. Information conveyed is accurate, but it is random and cryptic, and presented in no particular order...

What exactly is valid as a "target" in the context of this mirror isn't specified. Does a character need to be able to see the target as the mirror is being used? Or does the target merely have to have been seen by the character at some point? Or can the target be any specific object the character knows about, even if they have not actually seen it? Since this isn't answered in the description, I think it would be reasonable for a DM to make a ruling that best fits the narrative.

And if you don't want to allow the character to target an object they have never seen, you could require a two-step process. Step one, the mirror is used on the scroll in their possession, and provides glimpses of the scroll being created along with its other half. Step two, having seen the second scroll in the vision, the character could then target it with the mirror, and perhaps get a glimpse of who last wrote on it.

Answer 3

You have a few options. The problem is that you possess the "affect", not the "artifact". Which is to say, you can examine your scroll all you want, but it is just showing the after affect, but you don't have the actual artifact that was used so your forensics need to take a different approach.

Think of this like someone using the Skywrite spell. You can see the smoke message, but you have no idea who cast the spell.

So instead of working on the scroll, you need to seek outside help.

Catch the writer in the act

If the writing is happening in real time, you can use means to see the writer. So as they write, you spy on the other scroll to see who is writing.

• Clairvoyance - This assumes that the other scroll will be written from a specific location, such as an office. However due to the 10 minute casting time, this will only work if they are a very slow writer.
• Scrying - Also 10 minutes, but can be any location.

Since there is the 10 minute delay, I would suggest finding a means to have them write more, under the pretense of clarification.

Ask a friend

• Divination - Ask if who you believe is writing the messages will write the next message you receive. So long as you get a message from afar in the next 7 days, you should know. Make sure to add the clause about receiving. Otherwise, the next message could be written by someone in the party and invalidate the results.
• Commune - Ask three questions to validate the sender: Was this message sent by Agent X? Was Agent X coerced by by threat or by magic to write the message? Was it written by the Agent X we have met, and not a clone or other imposter? This is probably your most reliable course of action.

For future secure transmissions

Two-factor authentication

Come up with a secondary means other than the scroll with which to communicate. So a Sending spell to confirm the information, sending stones, a certain signal in the sky, or a courier with a predefined code.

Encryption

Use a secret to ensure that only the correct sender knows how to write the letter. This can be a sigil that changes based on the message (a symbol of fire means stealth for instance) or the time sent (use a waxing moon in the morning and a waning moon at night), cipher text, or just write in abyssal (most spells let the caster read or understand spoken languages, but not write).

Challenge-response

After the initial message is sent, the receiver replies with a code phrase, to which the initial sender needs to reply back with the correct response. So someone sends a message to your party. Your character writes back, "Sounds good. We attack from the heavens." Unless the sender knows the the return phrase is "And blood will shoot from the abyss." you can have a good idea of if the sender is legit or not.

Now, with all that, there are spells and creatures that can read minds, clone someone, and other nefarious possibilities. But this will give you a better sense of security.