how do you know if a piston will need to be replaced by a broken timing belt

There most likely is cylinder head damage, but pistons can be damaged as well. When the belt fails, the valves can be struck by the piston. this bends the valve, and can damage the piston. The only way to verify any damage, is to use a boroscope in the spark plug hole. This is a small camera that can find internal engine damage. If one is not available, the cylinder head will have to be removed to see the extent odf the damage. A bent valve will require at least a valve job, and maybe a reconditioned cylinder head. Piston damage in a vehicle of that age is a fatal wound, and the best repair is a remanufactured engine. If the car is in good shape, a new engine may be a wise investment. It is costly, at least a $3000.00 repair, but that is less than a new car. A used engine is a less expensive option, but the warranty on that would be short, at best.