You could check with a field strength meter, a VSWR meter, a throughline watt meter, a spectrum analyzer, for a myriad of other pieces of equipment. If you think it is for your VHF antenna, the easiest way is to tune your VHF to something such as a weather channel and simply unhook it and see if the signal dies. To test it to see if it's working properly there you should use a VSWR meter. Many people seem to think the quality of an antenna and transmission line effect only the transmitter but that's absolutely untrue. They affect receive and transmit equally. So that being said you most likely can verify it just by listening to a weak signal and comparing with someone else's boat listening to the same week signal. At least that will give you an idea as to how well it is working. But because it's not on the top of the mast it will not be a sensitive as your neighbor's boat. So maybe compare it with a handheld or a powerboat that has no mast to put the antenna on. All things being equal it should be more sensitive than your handheld because it does have a better antenna system.