Jack up trailer, rock tire in/out, if there is play then you need to replace the bearings. Spin tire, if you hear grinding, replace. Take tire off, spin hub by hand, if you feel grinding or lots of resistance, replace. Should be a standard check every time you change/flush the wheel grease each year. If you see milky grease when you flush, then you need to replace the seals and might as well do bearings too.
If maintained, they should not ever need replaced.. maybe every 50-75k miles which is more than the life of the boat trailer for most people.
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