Not entirely correct - the Kickstart in use might be loaded (Superkickstart or softkicked) - important for an eventual CPU upgrade.
Hold down both mouse button while switching on. If the boot menu gives you the choice of 1.3 and 2.0 you've got an old Superkickstart machine (actually 1.4), absolutely good for standard use, but incapable of running an 040/060 and some other hardware.
Otherwise, deactivate all bootable hard disk volumes and start the machine. The Workbench screen will show the actual ROM version (like adolescent wrote, but without disconnecting the drives). Amigakit was also right, but that way you'll see the ROM in use - everything that matters if you're not planning to upgrade the CPU. :-)
BTW: what's the price for the ZIPs from Magnetic Systems? (NYC?)