That is hard to do with out a testing setuip. The easiest ay to test the power supply is to swap it out with another working power supply (I would use a 650 watt PSU or better) and see if the problem goes away.
You would have to put a stress on the system. Then you could use an an electricians clamp meter to measure the amps. And then you could calculate the watts . But I am getting out of my depth here.