Calculating pot odds and knowing probability of catching cards you need is essential. But I think trying to calculate the probability of what someone else might or might not have could get you in trouble, because you never know what someone is going to hold with or lay down. Your best bet is to develop methods of testing the waters. If you have three besides yourself in the hand after the flop. If everyone stays or more importantly someone raises, then chances are at least one of the 3 have a K. If you are playing with people who know what they are doing, then they shouldn't be playing unless they have a nice pair or higher cards. However at this limit, you never can tell. Again making it difficult to try to calculate the probability of what someone else has in their hand.

You can use probability to determine the possibility that someone has something in their hand, but only before the first round of betting. That is no one folds. Once you had the human factor into it, no dice.