question

how do i figure out if my harddrive is 7200rpm or 5400rpm (for my parents' computer)

The easiest way is to get the model of the drive from Device Manager and google the number, as usually manufacturer information is not given in Device Manager. Once the type of drive is determined, the specs are a visit to the manufacturer's site away.

If you are unable to determine the model through DM, then simply pull the case cover and drive to determine manufacturer and model.