Some good ideas here. First the CB antenna should be straightforward to identify because it will be a different length than the VHF Marine antennas.
I assume there isn't a connection at or near the base of the antennas.
I like High Wire's suggestions.
Another method would be to get a small neon indicator light with a non-metallic housing, tape or ty-wrap it to the end of a wood pole, and then place it near each antenna while someone keys the transmitter of each radio with the transmit power set to High. You may need to move the light along the length of the antenna and conduct the test at night. The light should illuminate near the active antenna. Note that physical or electrical contact with the antenna is not required - just close proximity.
Example neon indicator: https://www.amazon.com/Indicator-Rep...6331831&sr=8-3