[BluffTHIS!]

If he is either a bluffer or would for sure bet an underfull here, then check/call most times, but bet small if you think he would make a full raise with an underfull or bluff. The problem of course is if he has a better A full where you want to lose the least to same in which case it would be best to bet a little more like 1/2 or 2/3 and fold to a raise. The only hand you lose value to by not betting is an underfull that would then check behind. So the more aggresive he is then I would check or bet small, and the more passive he is I would a little more probably planning to fold to a raise.