How do I know my harmonic balancer is bad on my 2002 Ford F-350 w-7.3lt. Diesel?

The transmission could be slipping have it checked-check the transmission fluid level and condition, also check axles

Check your belt and tensioner, if you see the tensioner moivng around it might be bad, if you have never changed it you should as belt life is normally 60,000 miles. If you do change it creplace tensioner as well. To check for bad balancer, draw a chalk line accross front of balancer, with engine off, startup and rev engine then shut off. Of cousr with a turob engine make sure yo let it idle down properly before shutting down, then see if chalk marks are lined up. If not the rubber has broken in the balancer.