How do you use counterspell fairly?

Not so much a question about rules, but:

If I (DM) start casting a spell, I describe that my character is going to start casting a spell. This is a trigger point for any of my players to cast counterspell. But they don’t know what the spell is, so they don’t know what spell slot to expend.

If a player starts casting, they usually say “I cast fireball” (eg), to which I, as a DM can start counterspelling.

But I feel like this gives me an advantage, maybe even a metagaming advantage, towards my players.

However, I, as a DM, don’t want to go “the evil wizard is going to cast fireball!”. That would be opening doors to metagaming as well.

How would I solve this and give my players a fair opportunity to do counterspell? Should I have the spellcasters roll arcana each time? Should I just gamify it and tell them what my evil wizard will be casting?

How do YOU handle two way counterspelling?

Edit: I am looking for a cheat-proof way of doing things: If my player doesn’t need to tell me what spell and spell-slot they’re using UNTIL my evil wizard identifies the spell, what stops them from telling me “oh you identified it, it was only a magic missile, I don’t expend a high level slot” instead of “yea your wizard got it right, it was an 8th level spell that they definitely want to counterspell”.