You can test it with power on as well. Be sure the drives are UNplugged.
Hold 1 pin against the chassis and other against 1 of the 4 molex pins.
You'll measure 2x 0V, 1x 5V and 1x 12V --> The 0V's are the grounded ones.
Just measure volts directly from the pins. If you measure between 2 pins and read 12V you have 12V, if you measure 5V between 2 pins you have 5V. If you measure 7V you have the 12V and 5V pins. --> if it reads -5V, -7V or -12V, turn your multimeter...