Knowing a wheel bearing is bad in your vehicle may not be as difficult to diagnose as determining which wheel bearing is bad. Most often, bearings will give off a telltale grinding noise in motion when in motion. Although this is not always the case, trying to ascertain the wheel bearing that is making the noise can be difficult since the noise and vibration travels across the entire axle. There are other ways to tell.
Test drive the vehicle to see if a noise is present when the vehicle is in motion. Certain bearings fail at different intervals and the percentage of their failure can dictate what type of noise they will make and when. Many bearings will hum or roar loudly while driving straight and intensify upon acceleration of the wheel, then dissipate when turning. Others may make a grinding noise when taking a turn and applying an axial pressure to the wheel. If a noise is prevalent on the test drive, it should be relatively clear which axle it is coming from which now cuts the diagnosis time down and can now focus on that one axle.
Place the car on a lift with the car in neutral and the ignition off. Bring the car up to waist level on the lift and spin the two tires on the suspected axle. In some cases if a bearing is at the point of failing badly, you will be able to hear the noise or feel the grinding while spinning the tire.
Check for any axial movement in the tire. This is placing one hand at 12 o'clock and your other hand at 6 o'clock on the tire and move it up and down. While some cars with adjustable bearings may allow for a slight axial movement, cars that have a hub bearing assembly allow zero movement. This could also be a telltale sign that a bearing has gone bad.
Follow the first three steps first and if you still cannot determine which bearing is bad, place a helper in the driver's seat of the vehicle. This test will only work on the drive axle side of a vehicle and not the other. In other words, if it is a front wheel drive car, it will only work on the front axle and not the rear.
Have the helper place the vehicle in drive while the car is elevated on the lift. With a pair of safety glasses on, carefully place your hand on the coil spring of the vehicle and tell the helper to accelerate to 30 to 40 miles per hour. Compare both front wheels and if there is a telltale difference between any vibrations or grinding noises between the two. As an absolute last resort, you can place an extra large pry bar onto the top of the tire of the drive axle and wedge it between the tire and the bottom of the strut tower or frame to attempt to keep the tire stationary while the helper accelerates to 30 to 40 miles per hour in order to allow the other tire to turn, then switch sides and compare the intensity of any prevalent noises. This test will require a seasoned technician to know what he is doing to apply enough pressure onto the pry bar to keep the tire stationary. This can be extremely risky if the drive axle forces the pry bar free from the tire out of your hands. You can cause damage to the car or injury to yourself or both.