I assume you're talking about finding out this info for a machine your own, correct?
Select "About This Mac" from the Apple menu, then click the "More Info" button. There should be a listing under the "Hardware" section (probably called "SATA"...it's called "ATA" on my PowerBook). Clicking that should give you a model number for a hard drive on the SATA bus. Google the model number and that'll tell you the model.