I think you will always have a hard time knowing someone has one, but you to have to think of the range of the player.
If you raise AJ and its a J, T or 9 high board, or worse even.....then you have to be putting all the low PP's in the other players range. Now this doesn't mean be scared of a set.....you can still C bet, but if a tight or weak player calls your C bet, start to be more wary.....you can maybe check back turn for pot control, or bet small again if you have improved.....but just don't go wild!!! Save your chips for a better spot!