Check if the belt abruptly slows down after a few minutes. Place the treadmill on the lowest incline. Walk at 3 miles/hour and pull out the safety key. As a rule of thumb, it should take two or three full steps before the belt stops.
Place the treadmill on the highest incline and walk at 3 miles/hour. The belt should continue to turn evenly and smoothly.
Check whether the belt is sluggish at the lowest incline but operates normally at the max incline; if so, it is worn and needs to be replaced.
Connect an 18-volt battery to the treadmill motor. A cordless drill battery is a good choice because you can easily connect the red and black leads on the motor directly to this battery. The motor should run at 1 mile/hour.
Connect a 12-volt car battery to the motor if you don' t have an 18-volt battery handy. The motor should turn very slowly, but you should be able to walk on it. If either step 4 or step 5 fails, the motor should be replaced.