look at the back of the psu there will be a lable in that lable there will be written that how much watt your psu has.:)
How much watt it has means nothing. It's the amps on the 12v rail and what temp the ratings are taken at....which is why we need to know the make and model.
A generic 750w psu MIGHT put out 500w if your really lucky. A quality 750w psu put's out a real 750w+.
Oh and it's the side of the psu, not the back.