D&D 5E When to declare sneak attack with mutltiple attacks?

[miniaturehoarder]

Agree it's roll an attack and choose whether to sneak, not roll all the attacks and pick the crit.

 

[the Jester]

The off-hand attack is a bonus action dependent on what you do for your standard action. You must fully resolve the standard action before you can even start the bonus action.

I agree with your first sentence, but not your second sentence.

For instance: say you have the Shield Master feat. My interpretation of it is that you can use the bonus action to shove with your shield that the feat grants you before making any attacks, but once you've done so, you're committed to the Attack action. You must Attack.

Even if it was part of the same action (per fighter), each attack is resolved in serial, fully and completely, before the next is begun. I would never allow a high-level Fighter to roll three attacks and damage at once and then decide which to assign to which of multiple opponents (the third attack was minimum damage, so I'll apply it to the kobold, but the other two go against the dragon).

Totally agreed with you here. I allow a rogue with sneak attack, a ranger with the +1d8 vs. a wounded target, etc. to declare whether they're using that feature on a hit, but after they roll damage, it's too late.