Normally the best way is to boot the machine and read the BIOS post screen. It usually will tell you the make of the mainboard somewhere on that screen. Reboot it several times until you see the info you are looking for.
Other than that, it's probably silkscreened on the mainboard someplace. You could also try motherboard Monitor, that might tell you the mainboard you have if it's at least 6 months old or more... I'm not sure how good it's detection routines are though.