Timing Belt Replacement or Whole Engine replacement

My car started making an unusual noise then just stopped on the highway and
would start (though the engine turns). My mechanic tried to start it but told me
that the timing belt is gone and needs replacement and most of the time the
engine needs to be replaced because once the timing belt breaks the engine
goes with it also. So its either I replace the timing belt and hope the engine is
not affected or if it is then i have to pay for another labour for engine change
and buying a new engine. How do i know if the engine is not affected already?
Thanks a lot!!!