We have bearing buddies (or a clone) on our trailer. In spring, or before a long trip, I pump in some grease, then jack the trailer up and give the wheel a spin. If it spins nicely, no grinding, no sticking, no wobble, I consider it OK.
During the season, I check the bearings after 30 to 60 min of highway travel. If the bearings are still cool, things are OK. Another thing to look out for is for grease being thrown out of the bearing on the inside of the rim, this indicates that a seal has blown.
In three years, I've replaced one bearing because its inner seal had gone, and the other I simply disassembled, cleaned and regreased because it was still in great shape.
Here's a great step-by-step writeup on changing a trailer bearing.