Get a hydrometer and use it. It is the only reliable method for testing deep cycle batteries. Hydrometers test the specific gravity of the electrolyte and shows how active your battery chemistry is.
Caution: wear rubber gloves and protect eyes and clothing anytime you may be exposed to battery acid!!
Use only distilled water! A battery should be filled within a ¼” from the top. If your battery seems dry fill it with DISTILLED water. Non-distilled water may contain minerals and chemicals that reduce battery life and effectiveness.