To determine if you need a new timing belt, look out for the following signs: decreased engine power, overheating, vibrating or shaking, trouble starting the vehicle, squealing or rubbing noises from the belts, ticking noise coming from the engine, leaking oil, and check engine light. You can also check the CarFax for a repair history report, ask the previous owner, or look for a sticker under the hood indicating it has been changed. If unsure, have a technician inspect it.