Your wheel bearings are critical to your bikes running, they can get clogged up with mud, sand and dirt in the normal course of a ride and this combined with the grease in them can turn into a really good cutting compound, so rule #1 is keep them clean, greased and free of pollutants.
You should check your wheel bearings on a regular basis in order to minimise wheel and axle damage. Your bearing should fit tightly into the center of the hub. The outside of the bearing should turn with wheel hub and the inside will be fixed, non moving and aligned with the axle, ie nothing should turn except the ball bearings.
If there is any side to side freeplay then the bearings need replacing. If the bearing is loose in the wheel hub then it has "spun" and the hub will need to be replaced or sleeved and a new bearing installed. A good engineering firm can sleeve your wheel hub, make sure you provide them with the new bearing so they can measure it all up.
The most important thing though is that the bearing needs to fit in the wheel hub and on the axle with no other movement except for the ball bearings turning, there should be no up and down or side to side movements.if you have any kind of these or you are not sure about the bearings then get your mechanic to check it out for you.
When you get your bike serviced make sure the wheel bearings get checked along with the brakes and brake pads.