Not easy, without a dedicated power supply tester. You need some method for loading each separate ouput up to its maximum (so, for example, if your PSU's +5V DC output is rated at 10 Amps., you need a resistor or dummy load of approx. 0.5 Ohms) The same goes for each of your outputs. Then measure volts x Amps. for each output, add them together, and that will give you total Watts.
So, as you can see, quite complex and not really practical for most of us.
If you are really sure that your PSU is overheating and causing the computer to shut down, I can only suggest replacing the PSU. 550W should be OK for video work, unless your PC is a VERY high performance machine. I wonder what is the brand of your PSU? I suspect that its 550 Watt rating may be optimistic?