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.
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.