Easiest way would be to open the machine. Big circuit board in the middle -- motherboard. Along the back edge there will be a row of connectors that line up (ish, offset ~1/2 an inch) with openings along the back of the case (might be covered). There should also be expansion cards already plugged in.
Then find the video card. It's the expansion card your monitor is plugged into

might not exist; could be on the motherboard.
If the connectors are a range of sizes but look to be the same rough shape, it's probably PCI Express. Some will be ~1inch long, the one your video card is in would be longer.
If all connectors are identical in color and spacing, including the video card, it's PCI.
If all but one connector are identical in color and spacing, while the odd man out is the same size but different color and inset a bit, it's AGP.
If you can take a piccy and post it we can very easily let you know.