Use a volt meter.. I wish I could find my outlet tester but I can "Design" it for you
IT is a simple 12 volt accessory pulg (MOre on this in a bit) with a 500 Ohm Resistor connected to the positive pin (Center pin) and a tri-color 2-lead LED between the resistor and ground.
Using a known properly wired outlet hook it up so the LED glows GREEN
Solder and close up.. Now plug it in
NO light = No power
Green - Good
Red - Reversed
Yellow - AC
Made this thign back in the 70's when they started making cars with backwards sockets.