Very easy to find out. Unhook all the cables and then set the tower on a clean surface, there are usually screws or thumbscrews holding the side pannels into place(when your look at the front of the tower take the pannel on the left side off) unscrew those and pull the pannel off. BEFORE TOUCHING ANYTHING ground yourself on something metal like a fosit or the inside of your case(not the inner parts just case) Now look a the expansion slots, if you see a slot that is set further back than the others it is a graphics expansion port - if its the same size roughly as the others but setback its AGP - if its longer and setback its PCI express. Now put the side pannel back on - hook up the cables and you're good to go.
