When the belt is worn down you'll hear it skip on hard acceleration and deceleration. This is what it will sound like:
If your board is making the noises in the video above, follow the directions below:
1) Determine which motor's belt is slipping. With the board powered off, turn it upside down and hold the motor still with one hand while attempting to turn the wheel with the other. If you can get the wheel to turn without the motor turning, and it makes loud ‘clicks’ as you do it, the belt may be skipping over teeth on the motor pulley.
See the following video. It shows someone trying to rotate the motor and wheel against each other.
If necessary, repeat for the second motor and wheel.
2) If a new belt doesn't improve the issue, follow these directions to evaluate your pulley to see if its worn down.