Short answer, pay attention!
How you tell a donk probably has alot to do with your definition of the term donk. My understanding is that it refers to a player who is to stubborn (like a donkey or mule) to lay down a hand when they should know that they are beat. There are lots of bad players out there. Not all of them are donkeys. There are a lot of unorthodox players out there who play a wide range of hands, that are quite successfull just because they are so unpredictable. I don't think that makes them donkeys, especially if it works for them. The donkey could just as well be the guy who calls a big pre-flop raise with pocket Qs and then refuses to fold to an all-in bet when the flop comes A-K-K. Not because Q-Q is such a donkey hand to play, but because he should have known he was probably outflopped and not been too stubborn to his Qs down. I guess what I'm saying is that it's not necessarily about the cards as much as how they are played. A-K is by no means a donkey hand. But when the flop is middle cards, and the turn puts a third heart as well as a possible straight draw out there you might need to consider folding to bet. At some point you have to consider that your "Big Slick" has shrunken down to just A high and let it go. But that's just my opinion.