Which iMac? How are you booting it?
If you already have an older version of Mac OS X on the drive, boot up to it and try to run the installer ... it will usually complain if it's not compatible.
If it doesn't have a FireWire port (which is usually the quickest way to determine it), you can't run it "natively." But you can "force" the install with a little help from OWC's
XPostFacto.