Is there a way to detect an NPC's class without them knowing that I'm detecting it?

Question

The description of the detect thoughts spell says that when attempting to probe deeper into a target's mind (which I assume would be required to detect their class):

the target knows that you are probing into its mind

I want to know whether there is any way at all to determine an NPC's class without them knowing in 5e, e.g. by adding other effects from spells or magic items, or by a different tactic entirely.

I would be OK with answers that have limits such as detecting some classes but not all.

Answer 1

There are no rules that would explicitly provide a way to do this, because D&D doesn’t want to make a stand on whether or not classes are things that characters are aware of and know themselves. Some things—paladin oaths, warlock patrons, etc.—are pretty explicit and obvious, but others, particularly fighter and rogue are very generic and can lend themselves to a wide variety of characters, occupations, and fighting styles. There is no particular reason why they would necessarily see any commonality between them.

Which therefore means that class may not be a thing to know. That may, depending on your campaign, be purely metagame information. It’s entirely plausible, in fact, that even things like oaths and pacts are not as neat and consistent as laid out in the books, but rather more unique to the individuals involved. It is very reasonable for, say, a sorcerer/warlock to say that all of their power comes from the patron, just that it’s a different sort of pact than a pure warlock has. And so on and so forth.

Since D&D doesn’t want to dictate how any given table handles this, they cannot write mechanics that rely on it being “real” in-character information, so no such detection spell could be written.¹ If classes are real, known concepts within your campaign, you will have to ask your DM for details on how you can know and recognize them in character.

1. _{This applies to a lot of things in D&D, and always has, though 5e is arguably trying to go farther with it than other editions. However, they aren’t doing so perfectly—while they often say they support a variety of playstyles, they sometimes seem to forget that and write mechanics that presuppose one particular style. But this is one particular case where they haven’t done that, and personally I suspect they won’t.}

Answer 2

Battle Master Fighters know (some of) this information

The Battle Master Fighters gain the following feature at level 7 (PHB, p. 73-74, bold added):

Know Your Enemy

Starting at 7th level, if you spend at least 1 minute observing or interacting with another creature outside combat, you can learn certain information about its capabilities compared to your own. The DM tells you if the creature is your equal, superior, or inferior in regard to two of the following characteristics of your choice:

• Strength score
• Dexterity score
• Constitution score
• Armor Class
• Current hit points
• Total class levels (if any)
• Fighter class levels (if any)

So you could discover if the NPC had class levels and fighter levels: this isn't foolproof. If they have none, you might be told "fewer" by the DM and not given more information. But if they have as many or more class levels or fighter levels than you, you'll definitely discover that.

Answer 3

So, there's a technique used by some called "Profiling." In short, "the act or process of extrapolating information about a person based on known traits or tendencies." (Source) The process usually requires some degree of Perceptiveness, Insight, and general Wisdom to be able to piece together from characteristics seen what the person may do and extrapolating from that what Class they may be. While Profiling is an art as much as it is science, you should also come up with alternative possibilities in case your first guess is wrong.

To a degree, what you're looking at is a character like the BAU in Criminal Minds, Sherlock Holmes in (well) Sherlock, or Patrick Jane from The Mentalist and you would want your character to make use of the tricks seen in these shows.

This gets you from a real-world perspective, but now we need to translate that into D&D 5E. Of course, you can ask various questions to your DM in order to try and pull information and prompt rolls, but this may seem inefficient. "What are they wearing specifically that I can see?" DM says, but doesn't mention footwear "Are they barefoot or...?" DM answers, slowly getting annoyed "Do they have any weapons on them?" Yes/No "Are there any noticeable scars or calluses on their hands?" Roll Perception: Yes, a scar in between his index and middle finger/No/You can't see the hand clearly enough (If Yes:) "Oh! He's probably an archer!" (Otherwise: "What is his stance and posture?" ad nauseam That helps at least. Combine that with probing questions combined with Detect Thoughts, you could easily exfiltrate information about them that they wouldn't normally give away so easily.

Courtesy of Roll20

Questions verbally directed at the target creature naturally shape the course of its thoughts, so this spell is particularly effective as part of an interrogation.

Even if it's just a casual conversation, you can still learn this information, if you're skilled enough to ask the right things and if you or your character are wise enough to puzzle together the pieces you're given.

Combine this with the information you can gather from being a Battle Master Fighter (as was earlier mentioned) and combine it with the persuasiveness of high Charisma, you could effectively learn the general aspects of the character, even if you may be missing chunks of information. Even if you do fail to learn the specific class build they have, you can still learn a lot of other useful information about them that it should make up for it.

That said, never assume the DM will cooperate with you as you do this. They don't have to play along. They can limit the information you get or punish you for being too nosy. Also, this hardly constitutes as RAW. While a lot of it can be expected to work (like Battle Master), it's still in enough ways interpretation of what 5e allows that you shouldn't take it for granted if your GM says "no".

It's also worth noting, if you want this to be effective your character has to be MAD (multiple ability-scores dependent). It's not that it can't work if your ability scores are a little low, but the higher your Wisdom and Charisma, the better, and if you're a fighter, you want high Strength too. It's not an optimal scenario.

Answer 4

Generally no, but sometimes you could get partial knowledge of this

Only because the OP said it would be open to limited solutions, we could consider using Detect Magic

you can use your action to see a faint aura around any visible creature or object in the area that bears magic, and you learn its school of magic, if any

A liberal DM might rule that a spellcaster counts as a "creature.... that bears magic", but it's more likely that the DM rules a spellcaster would have to be actively casting or concentrating on a spell (such as Disguise Self) in order count as "bearing" magic. But in such a situation you would know that they are a spellcaster, though in 5e this doesn't narrow down class very much, considering things like Arcane Trickster and Eldridge Knight.

Furthermore, almost all wizards have an Arcane Tradition that dedicates them to one school of magic, so it could be argued if they are a wizard, you would pick up their school, and since no other classes have a school, you could infer they are probably a wizard.

Finally, via the same spell, you might detect a magic item in the person's possession, and then if through role-playing you could get enough observation of it to make a successful Arcana check, then you might know if it requires attunement by a certain class, and again could infer (though not with certainty) something about that person's class.