All PSUs are built to handle (using 650w as an example) 650w from the PSU itself, then if it's a goldrated PSU it draws 722w from the wall at 650w.
Meaning that the 650w labelling is what's drawn on the internal side, not the external.
EDIT: But if you want to check, you can buy a special something-something (English, blah!) reader that just shows you in an LCD-display how much watt is running through it. You then plug your psu directly into that small reader that you in turn put into the wallsocket.