I'm not a mechanic or anything, just a regular jane with a '94 Camry that is closing in on 200thou. I don't think it's really possible to tell when a timing belt needs to be replaced. I replace per the manual every 60,000 miles. Having a belt break is not something I want to experience! Good Luck!
All the above answers are the best to go by-some tell tale signs? hard starts,sluggish acceleration,rough idle. The belt is what keeps the crankshaft and valves working in a timely manner so that the proper timing of the spark is also maintained. As your belt wears-usually streach-the valve timing is late causing the above as well as poor fuel mileage too. Although the labor cost may be high the cost of the belt itself is around $40-a cheap fix compared to the towing cost and inconvience if and when you get stranded in possible 20 below temp conditions.