If you're not low on coolant and the temp gauge can be trusted, I think it's safe to say that if you reach 120C then the pump is not doing its job and you don't need to test it! Replace it before your engine melts!

(I'd use a 2nd thermometer, after it's shut off put it in between some rad fins, to make sure the gauge is right, if it isn't showing any symptoms of overheating such as boiling over)