I carry an entire replacement hub assembly when towing.
Never want to have to do a bearing job again while on the road.
When a seal goes and starts to sling grease, it's time to pull the hub.
Twice a year I'll jack up the axle and spin the wheels. If they don't spin
freely, or makes a grinding sound/feel, it's time to rebuild/replace.
Important to note...I don't launch my trailer, hubs/bearings last a long time
if not submerged every trip to the ramp. If I was sinking my trailer
every time I launched the boat, I'd be checking the spin 4 times a year.
When the weather is bad I check the bearings. This is due to the fact that I won't be fishing that day. I pull the wheel assembly and look at the bearings and observe the amount of grease on the bearing also the color of the grease. Also look for pitting of the races and bearings. Look for tolerance between the bearing and races. This can best be judged if you have a new out of the box bearing and compare it with.