[morzh]

It is simple: you just take your DMM, make sure the black lead is plugged into the ""Common' socket and the read into "V/R" (or whatever it is called), set your DMM to "Volts", hook the black (common) lead to your chassis and your Red - to the other side of the cap, that is not connected to chassis.

if you see the value shown with the "minus" sign then your polarity is in backwards and the Plus voltage is on your black. If not then your plus is on your red.