Just a note in case anyone is worried I’m adding a mage to every encounter, I very rarely use counterspell against my players; it’s one of the spells I consider to have high “fun-ruining” potential.
I’m struggling a bit to decide on how to handle this interaction in a way that feels fair. From my understanding RAW, a character doesn’t know what spell is being cast. I think you can use your reaction to make an arcana check to discern it, but of course then you can’t counterspell it. For enemy spellcasters I generally describe what’s being cast, instead of naming the spell right away, but it can slow combat down, and is a bit one-sided since when a player casts a spell they lead with “I cast X”. This leads to an imbalance where I’m aware of what’s needed to counterspell something while the players are not, and can cause some awkwardness trying to decide how to play around that without metagaming.
I can think of a few different ways to handle this, each with its own drawbacks, but I’m curious to hear what y’all do at your tables!
I personally only give NPC casters spells that fit them thematically or have in game reasons for having that spell. If counterspell is one of those, then I will use it to the full extent that an accurate portrayal of that NPC allows.
In your situation, if you are having the players roll to recognize spells, an easy solution, albeit one that slows things slightly, is having the NPC do this as well. I personally don’t bother with using a reaction to recognize the spell I just call for the check if they ask (within reason of course). The roll can be made behind the screen, but it should help things feel more fair for the players. I know most tables will use an arcana check with a DC of 10+spell level. Some tables will just have the caster roll against their intelligence score. Roll it however you feel is appropriate.