Checking Battery Water Levels

[Eesctom]

Anybody got an easy way to check the water level in the batteries without taking the batteries out of the boat? One of the batteries is sort of tight and its a real pain to get it out of there. Was wondering if there was any kind of battery dipstick, or other way to check water levels while keeping them in place. I wish they had something like the Coors light can where it would turn blue to show you the water level from outside.