The first sign of failure is seepage. They usually break off on the inside at the threads. Which is the thinest area of the fitting.
Wire brush the fitting and look for a rasberry color. Use a pick to see if it goes below the surface. Replace it, if so.
BTW There should be a proper sized plug
within reach at each and every thuhull. The small end should be slightly smaller then the inside diameter of the fitting. Chances are that the rotten fitting will spread apart while tring to drive the plug
in, usually to the backing nut.
Marlon fittings get broken by impact or over tightening. I have yet to see one actually fail.
Nylon fittings (white) should always be above the waterline and have the same problems as the Marlon except the ultravilot light will waste them. They start getting chocky and micro cracks.
Stainless fittings are for special applications and should never be used below the waterline.
Inconel is just plain too expensive.