I carry an entire replacement hub assembly when towing.
Never want to have to do a bearing job again while on the road.
http://www.trailer411.com/trailer-axles.html
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.
How to...
http://www.trailer411.com/trailer-maintenance.html