Sorry I should have added more detail. Take the cover off and look for signs of corrosion/leaking around the electrolytic caps, or signs of bulging.
Caps made in 2000 or later have a much better reliability in terms of aging on the shelf than those before.
Assuming it passes your visual inspection then use a variac to raise the voltage from 0 to 120V slowly. Turn the power off at the first sign of trouble.
Also, connect sacrificial speakers for the variac test. Any weird sounds and stop your testing.