Over-Running Pulley - How to test: https://www.gatestechzone.com/en/problem-diagnosis/accessory-drive-system/oap-function-tests Be aware, your car may not be fitted with one but it is good to see if it has and test for defect, if fitted.
Harmonic Balancer - How it works: http://auto.howstuffworks.com/harmonic-balancer.htm These can be damaged by incorrect removal in the past, and you would see the pulley deflecting but easiest way to test is to remove the belts and run the engine to see if the vibration has lessened.
Timing Belt: If in doubt, replace as a matter of urgency if overdue before it fails and destroys the engine