How to check if a high capacity, low voltage electrolytic capacitor is functional after long storage?
Put your power supply on it with a low current limit, limit the volts to 50V and watch the voltage. The volts should start at a low value and then increase to a high value. If you can set the current limit at a known low value you could even time the voltage rise to give you capacitance V=I.t/C. Then check for leakage put 45V on it and check that the current drops to a very low value. 

Checking the ones in the amp without removing them will be much more difficult as they will have components around them and you could damage other things if you try this. Check that non of them are swollen or leaking or have split tops then power it up and check functionality.