In the Actions in Combat section you have a SEARCH action (PHB p. 193) which includes a description with Wisdom (Perception) check.

When you take the Search action, you devote your attention to finding something.

It is probably on purpose that this is quite vague.

I think that what you are describing is a Search action. The characters wants to see whether the creature has scale that increase its CA, has teeth dripping poisons, has large ears and piercing eyes... maybe a focus symbol that could mean the creature can cast spells as a wizard or a cleric.

However, I would not give my players any detailed information such as

"Oh! By the way this mummy is immune to non magical weapons."

because such information cannot be inferred until you notice that your weapon had no effect (you will know if it did not have any effect on a hit, however, if it does only 50% damages, you probably do not know right away, you may notice after a few hits though.)

I had a Paladin use a normal weapon to attack Gargoyles and noticed nothing happened. He then used his reaction to send radiant damage, nice move!

In the Actions in Combat section you have a SEARCH action (PHB p. 193) which includes a description with Wisdom (Perception) check.

When you take the Search action, you devote your attention to finding something.

It is probably on purpose that this is quite vague.

I think that what you are describing is a Search action. The characters wants to see whether the creature has scale that increase its AC, has teeth dripping poisons, has large ears and piercing eyes... maybe a focus symbol that could mean the creature can cast spells as a wizard or a cleric.

However, I would not give my players any detailed information such as

"Oh! By the way this mummy is immune to non magical weapons."

because such information cannot be inferred until you notice that your weapon had no effect (you will know if it did not have any effect on a hit, however, if it does only 50% damages, you probably do not know right away, you may notice after a few hits though.)

I had a Paladin use a normal weapon to attack Gargoyles and noticed nothing happened. He then used his reaction to send radiant damage, nice move!

However, in all cases, the type of information you want to given for such a check should be a more detailed description of the creature and not technical information. For example, whether the skin is not flesh like, but actually scales, making you think it will be tougher to hit than a bear or such. You should not give information such as this monster has an AC of 15.

Actually fighting the monster would tell you what the AC is after a few hits that succeeds you should have a pretty good idea. Any other aspect would be the same.

As noted by nitsua60, a Battle Master fighter has to reach level 7 before it can know whether things such as the creature AC, Strength Score, Current Hit Points... are inferior, equal, or superior to his own abilities (PHB p. 73-74) and not only that, it takes him one whole minute to do so. So giving a totally free information (i.e. no action, information that should not be easily perceived) to players is certainly not a good idea!