Trailer wheel bearings should last 100,000 miles or more. They should even last 300,000 miles since wheel bearings on cars and trucks last up to 300,000 miles.
It can't hurt to take the wheel bearing caps off the trailer wheel bearings hubs though and take a look at the bearings and see if they have enough grease.
If you want to for peace of mind though you can take the bearings out and repack them and inspect them further.
If I had the wheel bearings out of the hubs though I'd go ahead and replace them with new ones since you'll have to replace the wheel bearing hub seal too.