look in the back of your desktop and see if you have VGA (blue, 3 rows of 5 holes) or DVI (white, 3 rows of 8 holes, and a slot which might have 4 holes around it) connector in the expansion slots.
usually you see one VGA and one DVI, or two DVI connectors next to each other.
you could open the desktop and see if a card with these type of connectors is pluged into the motherboard.