Stupid question: testing capacitors.

I know to get an accurate reading, one must remove a capacitor from it's circuit before testing. However, when looking for bad items in an amp for example, can one get by with leaving them on the board while taking into account doing so will introduce inaccuracy?
Generally, I look for excessively low readings as indicators of failure; will leaving them in a circuit cause such random results as to make them unusable?
If I'm going to unsolder a cap to test it, I might as well just replace it. Or is that what most people wind up doing anyway?
Doug Elick

If you are looking at series interstage caps, you can look for DC Leakage with a voltmeter or scope. Since capacitors aren't supposed to pass DC voltage, any voltage leaking through a bad cap can bias the following stage in such a way as to increase distortion or perhaps even cause the stage not to work properly.
If they don't leak, aren't in series or otherwise look fine, you would have to at least unsolder one leg in order to test it with a capacitance meter. There are too many variables to test these in-circuit usually.
When in doubt, change it out! When an amp is 20+ years old, it could probably use mostly all new caps anyway as long as you are in there!
Steve

Steve is absolutely right and in the case of power supply filtering caps, they can go off and make one hell of a mess as well as taking out your transformer, which in these amps, is hard to find and EXPENSIVE. It's that old ounce of prevention thing. Big electrolytics make a mess! Mike.