Jack up and rotate each wheel slowly, listen for slight grinding or feel for dragging or sloppiness as scott described. A bad wheel bearing usually makes a howling sound. It will often stop if you hit the brakes as that puts a side load on the bearing. I thought I had one going recently, turns out if you are running out of gas and the pump is sucking air it makes a howling sound, hit the brakes noise stops (was fuel sloshing back to cover pump inlet), run out of gas all engine noises stop but howling gets louder even with car not moving, go figure.