if the wheel bearing fails the wheel wont just drop off, it'll make a great noise when the bearing collapses but it'll stay on fine for many miles, and then seize solid when you stop and it cools.
months before that happens you'll hear a squeal, if its the passenger side it'll squeal when you go round right hand bends (and vice versa).
to check at this early stage you simple spin the wheel (with it jacked up) and listen for roughness. its often hard to feel for play when the bearing first starts to get noisey but you can hear the roughness when you spin the wheel.
if the car has done 100k on the original bearings they should be changed regardless, its a 5 minute job but you 'should' buy new nuts for refitting and you will no doubt need to buy a socket of the correct size (very big)
