Difficult to give an answer that will cover every case but, generally, they'll either work or they won't. The parts that will probably wear out first are the capacitor(s) and the fan (if there is one). Fan is easy to check, caps failing will probably smell pretty bad and cause the impedance to go funny (no resonant peak, and a severe rise from low mids upward). Catastrophic failure would be through overheating, and I'd expect there to be a lovely fragrance of burnt resistor present outside the box and clear visual signs of toastiness inside.
Did that help? I tried writing before coffee again...
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