How do you know it needs to be changed?

Toyota only lists timing belt replacement for "severe" operation, and at 60,000 miles. For normal operation (driven for more than 5 miles each time, no substantial trailer towing, idling or similar harsh service), timing belt replacement isn't on the maint.
schedule.
Actualy Toyota engines are desgined so if the timing belt breaks it won't harm the engine block, valves, pistons, heads etc. It is going to cost you the same to replace it after it breaks or before it breaks, however if you wait till after it breaks you have an added cost of having the car towed to get it fixed. My friend's '94 Corolla's belt didn't break till about 134,000. So you can replace it now so you aren't stranded or wait. It will not harm your engine if
the belt breaks.