Like any other bearing, you'll hear & feel it - they will typically moan or howl with the vehicle in motion, and the pitch of the noise can be varied by rocking the steering to cause the load on the wheels to change.
If you'd like to check yours, jack the car up until the wheel is several inches off the ground and place on jack stands - spin the wheel, there should be no "rumbly, grumbly" noises, there may be some drag and rubbing because of the disk pads - next grab the wheel at the top & bottom (12:00 & 6:00 O'clock) and try to rock it, there should be no play, do the same at the 3:00 & 9:00 O'clock positions - if there is play at both 6:00 & 12:00 and 3:00 & 9:00, you most likely have worn bearings, play at 3:00 & 9:00 without play at 6:00 & 12:00 will typically be worn steering linkages, play at 6:00 & 12:00 without play at 3:00 & 9:00 is likely to be the ball joint (this can be very hard to detect) or in the strut.