The Evocation specialist wizard's overchannel ability says:
When you cast a wizard spell of 5th level or lower that deals damage, you can deal maximum damage with your spell.
Does the "that deals damage" clause mean the wizard can decide to overchannel once they are sure the spell is going to deal some damage? i.e., that it is not counterspelled or evaded by a successful save, etc.
(This would be similar to paladins being able to choose to apply smite damage after they know they have hit, so this kind of post-hoc add on is not unprecedented.)
Best Answer
No, in this case you need to decide whether you're going to use Overchannel when you cast the spell.
The timing for using Overchannel is "when you cast", and a spell you can use it on is "a wizard spell of 5th level or lower that deals damage". This is merely meant to specify that you can only use it on spells that include damage as part of their effects (like fireball), as opposed to spells that don't deal any damage (like invisibility).
If it was meant to be decided after it had dealt damage, it would have to use wording like "When you cast a wizard spell of 5th level or lower and deal damage".