Disconnect the capacitor from the supply and test the
capacitor terminals with a volt meter, you should have the
same voltage at these terminals as the supply voltage the
capacitor was connected to (you should probably not leave
it too long before you test the capacitor as some
capacitors tend to discharge b y themselves over time).
or you can discharge the capacitor by shorting the
terminals WITH A RESISTIVE LOAD you should never short out
a capacitor with that sort of voltage, it will be
danegerous, by using a resistive load, it will take longer
but it will be safer at least.
once you have discharged the capacitor, most decent
multimeters will have a capacitive test function, just test
the capacitor at the terminals and compare the value with
the value written on the side of the capacitpr and this
will also give you an indication of the its condition..
otherwise, call an electrician...