The easiest way would be open it up and look.
You see all those white slots? Those are your PCI slots (yes, it may look a little different on your motherboard, or you probably have less of them)
If you have that black slot (could be green, yellow, orange, or even f-ing rainbow on your motherboard), that means AGP. It is roughly the same size but set back towards the middle of the board a little more & a different color.
NOTICE: your board will be rotated 90 degrees clockwise of that one, so that all those slots are in the lower left of the motherboard if you look at the computer right-side-up