Wheel bearings on a car last tens of thousands of miles. Quality trailer bearings will too, if you keep the water out and grease them occasionally. To inspect, block the trailer and jack up one side. Grab the tire top and bottom and see if it will move back and forth - it shouldn't. Then spin the wheel and listen for noise (rumbling or grinding) - there should be none. Going on a long trip? Check the hubs for heat occasionally. My previous boat trailer had 25 years of use with the same bearings. No problems.I suspect one reason that most shops always replace bearings as they don’t want to depend on their employees to inspect the bearings.
Now brakes, on the other hand ...