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!