Remove the back cover of the washing machine and look for the belt or a place where a belt might have been. It should run from a spindle on the motor to a spindle on the tub. Tip the washer forward and check the bottom of the machine if the back has no belt access.
Check the belt's tension by plucking the belt. Listen for it to make a "ping" sound. Test the belt for play by moving it from side to side; it should give slightly but no more than 1/4 inch in any direction.
Inspect the belt for damaged or worn spots by rotating the belt slowly and using a flashlight. Replace belts with worn rubber or excessive cracking according to your washing machine's instruction manual.