My mechanic tells me that there is no way to tell whether my timing belt is good and will not fail in the near future. Is this so?
Yes, there is no way to tell if a timing belt is good. You can only tell if they are bad. The part of the belt that fails is covered by a cosmetic layer of rubber.
The inside of the belt is made of fabric that gives the belt its strength. The fabric can't be seen so you can not tell if it is good by looking at the cosmetic rubber covering. The normal rule is the mileage or four years whichever comes first, but do check with your specifics from the manufacturer because in recent years timing belt change intervals have increased.