The Toyota timing belt is one of the few internal engine components that are not lubricated by the oiling system on your vehicle. This is a polymer component that will show signs of wear just like the drive belts. This will include stretching, fraying and cracking of the timing belt.
An inspection of the Toyota timing belt cannot be accomplished without disassembling the front of the engine. The timing belt cover must be removed which will require most of the components on the front of the Toyota engine to also be taken off. Because of this, it is recommended that this engine component be replaced according to mileage that has been placed on the engine rather than an intense and laborious visual inspection.
After the engine has reached 100,000 miles, the timing belt on your Toyota should be replaced. Even if the cover is removed and a visual inspect does not indicate excessive wear on the polymer belt, it should still be replaced. The damage that can be inflected on the inner works of your engine is massive if this polymer component does break in the future. The damage will include the valves and pistons coming in direct contact with each other damaging them to the point the entire engine will have to be rebuilt or replaced.
100,000 miles has been accepted as the standard replacement interval for most timing belts, and any vehicle on the road for that long will have some internals that also need to be fixed or replaced. Performing these services all at once will save the Toyota owner from having to take their car or truck in multiple times to have work done on the engine.