Usually a few trips before they go they will begin to rumble and then squeal and then bang-bang. Much like a u joint failure. First indicator would be lift the wheel off the ground and see if there is up and down movement. Easy nough to pull the axle and take a look. Most of the time the trouble will be the bearing spins on the axle and it's no longer a good tight fit. At that point your axle is pretty much toast.