How to test a used amp?

You mentioned a multimeter,checking each speaker output on AC and DC setting sure can't hurt.On a amp that has a slight hum,I get under 1 volt (it needs a new cap).A good amp was giving almost 0 millivolts with no speaker load.A tech may have better ideas.Tube amp people,make sure you have a load resister on.

Both good suggestions, thanks.

Mark

You can also short the inputs, NOT OUTPUTS, and, with speakers connected, that will tell you the ambient noise level of the amp.