How long can equipment go unused before powering up runs the risk of damage?
If you're worried about it then get a capacitor ESR tester, desolder a few from the board and test the values of the caps and make sure they match the markings on the package. Also test the ESR. If you can find a datasheet for the caps, then you might find nominal ESR values there or find an equivalent capacitor in that range. If the ESR values seem high then replace the caps.

If that can't be done then find the voltage of the circuit and unsolder a few and test them with a supply, if they are leaking a lot of current, then don't use them. 

It's probably better to replace them anyway if there is a question, especially any caps that could have large currents flow through them.