The simplest way is to get yourself an AC clamp meter and measure around the shore power cables of your boat and other boats close to you. The clamp is just put around the shore power cable. It should register zero amps, i.e. all the current flowing to a boat is coming out again. If you see a net reading, some current is leaking into the water. Dangerous for swimmers, particularly in fresh water.
Make sure your boat has a galvanic isolator in the ground wire or an isolation transformer. Otherwise, your zincs/metals could be protecting someone else's boat.
The clamp meter will catch AC leaking into the water which is dangerous but typically does not cause corrosion as the current changes direction every 60th of a second.
You can also read potential/voltage differences directly in the water with a multi-meter and long probes. But this requires a bit of DIY and some knowledge of electrical concepts.