https://rpg.stackexchange.com/questions/46830/what-do-i-know-when-deciding-whether-to-cast-counterspell https://www.enworld.org/threads/do-you-allow-a-spell-to-be-identified-before-counterspelling.678789/ https://www.flutesloot.com/counterspell-5e-spell/ https://www.reddit.com/r/dndnext/comments/8syfdy/dm_asking_for_help_with_counterspell/?utm_source=share&utm_medium=ios_app&utm_name=iossmf

Counterspell should feel more like an instantaneous Dispel Magic. I think it's reasonable to expect that you could see someone casting Fireball (or see the Fireball start to take effect) and you react to the physical part of the spell to disable it.

XGtE:

Identifying a Spell​
Sometimes a character wants to identify a spell that someone else is casting or that was already cast. To do so, a character can use their reaction to identify a spell as it’s being cast, or they can use an action on their turn to identify a spell by its effect after it is cast.​

If the character perceived the casting, the spell’s effect, or both, the character can make an Intelligence (Arcana) check with the reaction or action. The DC equals 15 + the spell’s level. If the spell is cast as a class spell and the character is a member of that class, the check is made with advantage. For example, if the spellcaster casts a spell as a cleric, another cleric has advantage on the check to identify the spell. Some spells aren’t associated with any class when they’re cast, such as when a monster uses its Innate Spellcasting trait.​

This Intelligence (Arcana) check represents the fact that identifying a spell requires a quick mind and familiarity with the theory and practice of casting. This is true even for a character whose spellcasting ability is Wisdom or Charisma. Being able to cast spells doesn’t by itself make you adept at deducing exactly what others are doing when they cast their spells.​

An elegant way to handle Silvery Barbs, thanks to one of my friends:

The way it’s written makes it hard to stop a Crit. Because you’re supposed to use it after you know the roll succeeds but before it’s a Crit or not… which if you roll a Crit, you’re not going to ask if it hits or not… so basically if someone wants to use it, they should announce it after someone declares their intention to attack or force someone into a Saving Throw, but before they roll the dice… which if the roll fails, then you just don’t use the spell slot. And if it succeeds, you use it, Crit or not.