The average timing belt is designed to last roughly 100,000 miles. This time can lessen with excessive driving, such as averaging more than 10,000 miles per year. Checking for signs of wear and knowing the driving conditions and life of the vehicle can help you determine when the is best time to replace the timing belt. Because of the nature of the timing belt's job, essentially regulating the engine, repairing a timing belt is rarely an option and most mechanics will advise against it.
Open the bonnet, with the vehicle turned off, and locate the timing belt. Visually observe the belt, making note of whether there are obvious signs of wear such as drying, cracking or fraying of the edges. Also note the mileage on the vehicle's odometer; if the mileage reads in excess of 100,000 miles travelled and these are known to be the original miles on the vehicle, the mileage alone could be reason enough to have the timing belt replaced.
Start the vehicle and let the engine run idle. If the engine has started running hotter, or idling louder and possibly vibrating or shaking the vehicle, this could be indicative of a timing belt going bad. Additionally, some mechanics mention knocking or ticking noises from the engine when the timing belt is going bad.
Drive the vehicle in a small area, not on the street, to assess whether the vehicle is backfiring. This is an unmistakable gunshot-like noise that sometimes occurs, especially in older vehicles, when the timing belt is going bad. This occurs most often when decelerating. When the accelerator is stepped on, if the engine revs fast or slow compared to "usual" and relative to the pressure applied by the driver's foot, this could be a symptom of the timing belt being off. In other words, the timing belt is running too fast or too slow and is affecting the engine overall.