Testers like this are available at every hardware store. I carry one with me on every job and can cost less than $10. It tests an AC receptacle for reverse polarity and open ground. It is such a convenient thing to have when troubleshooting AC circuits, I recommend every boat carry one.
Its very simple to use; if all three lights light up the circuit is good. If any fail to light the fault code is written right above. Apart from the circuit not working what you particularly want to make sure is that you do not have an open ground. With an open ground the circuit will work but the safety features of the ground will not.