With ALL power to the furnace turned off, reach in and try to move the blower wheel up and down. If you get a noticable movement in any direction other than in and out the bearing(s) are more than likely worn and the motor needs to be replaced.
And yes, the motor has a built-in thermal cut out in case it gets too warm. With power supplied to the motor (and it not moving) all that electrical energy that now isn't being turned into mechanical energy gets turned into heat instead...... it's called locked rotor amps. Once the motor cools off the internal switch closes and the process starts all over again.
When the bearing first start getting bad an indication "usually is" that when the motor first starts up you'll hear what kind of sounds like an unbalanced starting noise.
Hard to explain in words.