Some PSU info on this website
The easiest way to check if this PSU is a specially made Compaq item or a standard ATX is to compare it with the one in your own PC. If you have built PC's previously you will also know what the standard ATX PSU looks like.
Another PSU info site I have bookmarked is this
and this guy reckons its better to buy a PSU by weight - the heavier - the better the quality