You have to convert the RCA connect to a hot/ground wire. Buy a cheap 3 foot RCA cable, cut it in half, and seperate the two wires inside.
Attach the multimeter, hot&ground, turn on your system, and measure the voltage.
Hope that helps. It's also a halfway accurate way to test your amp.
Just make sure it's not mentioned on the box. Mine is only 2.5 volt, as advertised.