If I have Extra Attacks, when can I decide which one is a sneak attack?

So, if a character multiclasses to a point where they have multiple attacks in a round, I assume you can decide once the hit has been confirmed by the DM, but before you roll damage, whether you make the hit a sneak attack?

I ask because you may know an enemy is weak, and don’t want to waste the damage to finish it. I also want to confirm that you cannot declare sneak attack damage after you have already rolled your base damage, to finish off a still standing opponent for example. The “Is it dead? No? Oh! Then sneak attack!” seems dumb and nonsensical to me. I expect it to be like smite, decided on hit, but before damage is rolled.