Is there a spell, item, or other game feature that would give my character insight to an enemy's weaknesses and resistance?

Question

When I first heard of True Strike, I originally thought it was quite a powerful cantrip that would allow the castor to sacrifice 1 attack in order to be granted insight into the target's elemental weaknesses and resistances. It reminded me of my first campaign when I walked into it at level 4 and felt lost hearing my DM state "your attack seems to have done nothing" even when I knew it was a hit.

However, I now realize that was a false understanding of the rules. As such, I was wondering:

Is there a spell, item, or other game feature that would give my character insight to an enemy's weaknesses and resistance?

I am curious about any possible solution that would let a group know why the arcane wizard is not taking any damage in the middle of a fight, for example.

Answer 1

It's not a spell, but the Monster Slayer Ranger's Hunter's Sense has the desired effect.

The Monster Slayer Conclave, featured in Xanathar's Guide to Everything, gains the Hunter's Sense ability at level 3. This ability lets you:

As an action, choose one creature you can see within 60 feet of you. You immediately learn whether the creature has any damage immunities, resistances, or vulnerabilities and what they are.

This ability is blocked by effects that block divination magic (as in the spell Nondetection) and can be used a number of times equal to your Wisdom modifier per long rest.

Answer 2

This is a slightly different way of looking at it, but your history, arcana, nature and survival skills all offer you the ability to gain insight into your opponents weaknesses in different situations. Speaking directly to your example, an Arcana Check could tell you why the wizard was not taking damage.
Moreover a simple wisdom Chk can be used to:

"Get a gut feeling about what course of action to follow. " P178 of players handbook.

This also keeps skills relevant in combat, and gives rogues and bards a proper edge from having many skill slots.

A stone of good luck p205 of the Dungeons Master's Guide will give you a +1 to all skills.
If you want something "granted" to you outside your characters normal abilities, an artifact ability (p219 dungeons master's guide) is to have proficiency in a skill granted to you.. So you could work with your gm to have a magic sword or helmet that grants you specific skills for the purpose of identifying weaknesses in your opponents!

Answer 3

The (third-party) Cobalt Soul monk's Extract Aspects feature

It may also be worth mentioning the unofficial Monk subclass, "Way of the Cobalt Soul" developed by Matthew Mercer and used in Critical Role's campaign. The subclass is freely available on Mercer's dropbox (linked from this March 2019 tweet by Mercer), and an earlier version of the subclass is published in the Tal'Dorei Campaign Setting third-party book.

The Way of the Cobalt Soul monk's Extract Aspects feature says:

Beginning at 3rd level when choosing this tradition, you can strike to extract crucial information about your foe. Whenever you hit a creature with one of the attacks granted by your Flurry of Blows, you can learn the following attributes about the target: Damage Vulnerabilities, Damage Resistances, Damage Immunities, and Condition Immunities

Answer 4

There are several possibilities to get this information.

1. Hunter's Sense

A 3rd level feature of the Monster Slayer Ranger and the only ability I know of, that is explicit about those effects:

As an action, choose one creature you can see within 60 feet of you. You immediately learn whether the creature has any damage immunities, resistances, or vulnerabilities and what they are. If the creature is hidden from divination magic, you sense that it has no damage immunities, resistances, or vulnerabilities.

2. Abilitiy Checks

Intelligence (Arcana/Nature/Religion) can give you some insight concerning a creature. But it doesn't give you a guarantee.

Arcana

Your Intelligence (Arcana) check measures your ability to recall lore about spells, [...] the planes of existence, and the inhabitants of those planes

Nature

Your Intelligence (Nature) check measures your ability to recall lore about [...] plants and animals, the weather and natural cycles.

Religion

Your Intelligence (Religion) check measures your ability to recall lore about deities, rites and prayers, religious hierarchies, holy symbols, and the practices of secret cults.

But that involves only common knowledge about a creature's stats. It involves no new knowledge about a creature, it just depicts your character's potential knowledge about a particular creature. If your DM decides that your character never heard about that creature before, you cannot roll, of course.

3. Divination

Your magic and an offering put you in contact with a god or a god's servants. You ask a single question concerning a specific goal, event, or activity to occur within 7 days. The DM offers a truthful reply. The reply might be a short phrase, a cryptic rhyme, or an omen.

It's a bit of a stretch, but you could ask for example how you will defeat that boss to get a hint about their vulnerabilities.

4. Know Your Enemy

A 7th level feature of the Battle Master Fighter, that sort of solve your problem:

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)