Is this integrated or power amp. Either, you might need to rehabilitate these surplus amps by replacing their filter capactiors.
The surplus Onkyo stereo power amp I saw at the pier are quite old, about 25 years old. Maybe the latter models would not need it. At 3.5T, if it's still in good condition, then it's a real bargain as Onkyo power amps enjoiy audiophile status in Japan. Get their Integra series, the ones with large VU meters on the front panel.
To test, the dealer would just hook it up to any convinient CD player and speakers. I wouldn't give too much faith on the shop tests. Buying surplus gears is often a matter of trust and a gamble. You could ask to open the unit and check for signs of repair, corrosion, etc... Better still, just ask for a return policy if it fails at home.
