I dont remember that you know the spell casted before using your counterspell in Solasta... bad memory ?
Party members in range to see the enemy spellcasting automatically roll an Arcana check to identify an enemy spell. You do it with advantage if you already know how to cast the spell they're attempting to cast. You can still counterspell even if you fail the check, but the game will tell you that the enemy is attempting to cast an unknown spell instead.
I believe this is actually one of Solasta's homebrew rules done out of convenience, because I remember reading about how the normal rules for that mentioned something about how identifying an enemy spell counted as a reaction itself, or it was something that would prevent the person doing the identifying being unable to cast counterspell afterwards.