I was wondering if there's a way to tell the difference between an AGP slot and a PCI-E. I have to clue if my computer supports PCI-E video cards.

Physically an AGP slot is usually brown or black and set above and to the right of your PCI white slots.

A PCI_E X 16 slot is generally blue and set directly above the PCI white slots.

This scan will show your slots:

http://www.d-a-l.com/hardware-scan.php