If the capacitor is damaged then you really do just have to make a best guess. You can improve your guestimate by taking into consideration case size and capacitor function i.e DC blocking, decoupling.
If you just want to find out the value of a non-marked working capacitor there is test equipment that will measure capacitor values. A more fun way (though I haven't personally tried this I guess it would work) is to use a bench supply and a known 1% tol resistor. Attach the resistor and capacitor as a series circuit to the supply set to 5 V and switch on. Monitor the voltage charge on the capacitor, then use C=T/R where T is the time it takes the cap to charge to 63.2% of its final value you can work out the cap value.