Asuming there is no leaks
? outside of the hull
, the thruhulls are intact? any pitting or eveidence of corrosion
, scrape old antifouling or better sand back to bare metal and see if the metal is clean.
We talk about bronze, brass, marelon, plastic?
Inside, the first indication of a bronze valve or thruhull leaking or seepage is sometimes a green trace running from the base around.
Green clean bronze fittings are a indication of a healty fitting, scrape the metal with a knife and see if the bronze is pink in color or something like a gold in color, pink is bad..
Open and close and the valve and see if is smooth , any hard spot indicate barnacles
inside, or lack of grease.
deposits, a white thick goo outside of threads or around joints indicate corrosion and leaks
In doubt? replace it...Just few sugestions sure others are going to give you some hints... Cheers.