My high level players often face problems in combat. Enemy resists damage, something causes an effect, enemy's abilities are unknown. They often desire information about what has happened, i.e., "Why did I do no damage? Is he immune to Fire or is it something else?"
As I DM, I enjoy when players find out enemy information and respond accordingly. This falls under Improvised Actions, in the PHB.
Your character can do things not covered by the actions in this chapter, such as breaking down doors, intimidating enemies, sensing weaknesses in magical defenses, or calling for a parley with a foe. The only limits to the actions you can attempt are your imagination and your character’s ability scores.
However, if this information check is a full action, then PCs usually won't waste said action. They would rather do damage, or cast powerful spells, or anything else. From a player's point of view, I understand this approach. If the enemy goes down before the next round, all problems are solved, so this tiny bit of information seems quite irrelevant.
However, this tiny bit of information can be sometimes be crucial, i.e., "You notice a a crystal shining in the back wall, whose light seems to be shielding the boss and making him immune to damage!", and players need to deactivate this magical aura before engaging the boss. So, I homebrewed a rule,
Intellect and Wisdom checks (i.e., mental checks) can be used as a Reaction to an event that just occurred.
This has a cost for players (no longer able to counterspell, opportunity attack, etc), and allows them to gain information on what just happened. I would keep the information gained from these checks mostly limited, not the same as a full action spent on the check. So, for example, when the boss makes a necrotic attack, the players can use their Reaction to find out "Heh, is this an Undead?", and their Action to find "What are the weaknesses of this enemy?", using for example, a Nature roll (the PC might have found this creature in its past, and is trying to recollect information about it!).
So, bottomline, is this house-ruled mechanic balanced? Are there any classes that actually get a class feature to do such a thing, and therefore, it is not really fair for everyone to be able to do this for free? What are other options to allow players to get relevant information in combat without wasting actions?