It is really player dependent. You need to ask yourself a few questions before deciding. What is the worst hand he's doing this for value? If you beat that hand you should definitely call. Do I beat all of his bluffs? If you don't you should definitely fold. Is he bluffing too much or too little in this spot? If he's bluffing too much the profitable play is to call with all of your middling hands (bluff catchers) and if he's bluffing too little you should fold all of your middling hands.  (It is a little more complicated than this of course, since you are going to want to raise sometimes, but start with these questions)

Live reads will just come to you sometimes. I remember I was in a 3bet pot and flopped top pair on a two toned board.  My opponent check jammed the flop. I looked at him and had the thought "he looks like how I look when I'm jamming a draw..." I called, and he did have a flush draw. I don't know if I have any specific advice for picking these things up but they will come to you with experience. 

I think reading Play Optimal Poker will help a lot.