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.

I don't know of any tests off hand,but hook it up to speakers that are disposable ,and may have a fuse protecting them first.Even if the seller is honest,who knows what it went through in shipping.The monkey that grabs your baggage in a commercial comes to mind.Don't blow out your prized best speakers!♫