It happens to all of us, somebody bets into you out of left field, or raises your bet. If you are holding a hand that you believe may only be the second best hand at the table, most players' natural inclination is to fold. Before you surrender the hand to your opponent take a good look at his stack. This will often be able to tell you more about whether or not they are bluffing than any facial tick or profuse sweating.
If this is a cash game, and you see somebody with a small stack displaying this sudden flash of aggression, you can believe you are now facing a legit, formidable hand. Chances are your opponent has been just waiting for this opportunity to replenish their supply of chips and they are going to make the most of it.
In tournament play if the short stack is behaving in this manner the bet or raise is most likely a bluff. They are doing this because the blinds are killing them, and they would rather go all in and go down swinging than simply be snuffed out by the escalating blinds.
A player's financial situation definitely is going to have an effect on how they play a hand. Listening to what their stack is telling you may help you win a few more hands.