No easy way. Different light fixtures were used and no records of which boat got what. Anyways it would not be a loss of effort to go aloof and inspect the fixture. Check the gaskets, lens and bulb socket. Getting replacement parts could be impossible for some models so this would be the time to decide if a replacement is in order. Perhaps one designed to work with LED technology would prove a better long term investment. If everything is OK just get the bulb and use dielectric grease to protect the socket from corrosion. For future reference write down the make of your fixture in the owners log.