There's only a few standards, 5400, 7200, or 10000. Most likely, your hard drive is a 7200RPM drive, if it's relatively knew, say within the last 4-5 years. If it's a Western Digital Raptor, it's 10000RPM- which would have cost you a pretty penny. It should say on the hard drive. Or get the model number from the hard drive and do a Google search on it, you'll know then.