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!
The easy way we fixed it when we still played dnd is that casters know what the spells are, even if it’s not in their spell list. You know if the enemy is casting firebolt or finger of death. Consider that if it’s possible to counterspell counterspell, which only has somatic casting components, then probably there is sufficient time to tell what each spell is. This fixes the bad feel of counterspelling because the spell is known by having everyone do it. Everyone knows what the spells are so you can choose to be strategic and get the most out of your counterspell.
If you want to justify it, you can imagine that all fire spells have some fire movement or incantation, add to that something related to size or expansion, plus a bead of fire forming, that’s a spicy fireball.