Open the PC and look at it. See if it has a nameplate you can read. If the PSU is top mounted in the case, the nameplate may be facing the wrong way to see. In that case, you'd have to loosen the cables from any tie downs they may have, unplug them from their devices, and remove the PSU to read the nameplate.
Do you have a pre-built PC? If so, what is the make/model number of it? We might be able to track down the info that way.