What does it look like and how do I know I have one?

is quite a little port i think, smaller than usb..almost square i think

And its unlikely to be on the motherboard with the usb and ps/2 ports, it'll be on a seperate card, around by the gfx and sound and modem

This picture (http://www.compgeeks.com/images/300x300/net-r106t-unit.jpg) shows a 6-IEEE-1394 port repeater. The ones on PC will look like those. You should see 1394 Controllers in Device Manager if your system has it.