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...