Look at it?
Boot the computer and watch the POST messages?
Boot the computer and enter BIOS setup and check there?
Boot the computer into Windows and right click on "C:" to ask for the properties?
Boot the computer into Linux or FreeBSD and take a look at what dmesg tells you?
Boot the computer and log into KDE and run kinfocenter (K-Menu -> System, probably) and either look under "Partitions" or "SCSI"?
Lots of ways.