Reverse polarity indicators on the AC panel are a safety device. They can tell you if the dock power has been wired correctly or backwards. By the ABYC code (American Boat and Yacht Council), every boat should have one unless all your breakers are double pole (most are single). If the light turns on when you plug into the dock, disconnect and do not use the power, then tell someone at the dock. Using the power when the light is on is potentially fatal. Most likely you've never seen it on, so how do you know it works?
AC power has three wires - hot, neutral, and ground. They all must be properly connected to prevent safety hazards and for devices to operate properly. I was recently on a boat for another issue and noticed the Blue Sea AC panel was wired incorrectly. They had a ground bus mounted on the wall that connected the shore ground and load grounds together. The Blue Sea panel had the hots and neutrals connected to it, but it was not connected to the ground bus. This would have caused two problems. The metal panel was not grounded, which is against the ABYC code and a safety hazard, but also the reverse polarity light would never have worked. Is there a way to test it? Probably not. You could make up a backwards shore cord or adapter but feeding bad power purposely into the boat isn't the best idea. The bulb itself can be checked by disconnecting the wires and temporally connecting it to the proper 120V. The best way is probably just inspecting the AC panel to make sure everything is hooked up correctly.