Are they the old style bearing/race combo or a sealed hub.
If they're the old style, inspect the races for heat discoloration and scratches or water erosion. The "cones" or rollers shold be be tight If the cage holding the rollers in place is sloppy, I'd replace. It is best to replace the cone and race at the same time, along with the seal.
Sedaled hubs...put the tire back on, grag the tire at top and bottom and wiggle it. If the hub has any play in it the hub assembly should be replaced.