Donk Bets I've Noticed
First of all, you hope not to be both OOP and not the aggressor too often after the flop. Often is a product of -EV limping. 6max, or in full-ring even, you shouldn't be limping often, as you are missing information that might better define V flop or turn donk bets. There are many reasons you'll see a donk bet; some weak and some strong.
Donk bets:
1 - non conformist bet - Bet size anywhere from min-raise to push. Often a level 1 poker thinker and a newbie making his way up the ranks. V doesn't know or care about the rule in their system. They unknowingly just play their hand without regard to position or aggressor. Obviously a losing player overall, no matter what his stack looks like that session. Generally doesn't define newbies hand for you although bet sizing may help. They make their play, whether air bluff, semi-bluff, top set on a dry board, TPTK, or bottom pair no kicker when it is their turn, because well, it's their turn. Usually doesn't apply online past micro stakes, so you can easily disregard in most cases, therefore you can knock this option off the list except in rarest of donk bets.
2 - Blocking bet - Usually min-raise up to 1/2 pot; especially min-raise. Usually V wants to see that next card cheaply with a draw of some sort. Villain just let you know that he is weak, so do the opposite of what he wants, and make him pay with bad
odds to see next street.
3 - Feeler bet - Usually min-raise up to 1/2 pot. Villain has a piece and is likely worried that he may be dominated or way behind. Not wanting to bow out weakly with his small piece, he is testing the waters. Second use of small probing bet, is to check if aggressor hit the A or K on a A-high or K-high flop respectively, for a later turn steal attempt. If you want him to go away now, smite V hard with a goodly-sized bet. If you want a few more chips, 1/2 pot or less might bring him along. Slightly more dangerous, on a dryish board you can smooth call, and hope for him to lead out on the turn.
4 - Bet-three-bet attempt - Full-range from min-raise up to PSB. B3B is reserved only for Odin's fearless Berserkers. V bets into you with the guile of a weakish-looking donk bet only to come back with a second blow. This highly-powerful maneuver makes the deceptively strong check-raise look like a min-raise by comparison. Only the brave or stupid would use this as a bluff for a miss-read can get expensive. It screams SET or completed connectivity on the flop and can make the PF aggressor toss TPTK in the muck.
So as you see, there are many forms of donk bets with multiple purposes, weak and strong. Bet sizing and player observations can help narrow meaning down. HUD stats can often give you a high likelihood of the donk bet's true intent.