Mutation Testing for programs with an unknown output

Since the eventual aim of mutation testing is to detect faults with programs by finding test cases that would reveal the faults (by changing parts of the program and then verifying the output), it seems that only if the output is different for the mutants as compared to the original, can a fault related to that mutant be detected.

However, if the developer is not sure of the output of the program or if, for different test cases different values are indeed expected, how can she/he detect if a mutant has detected a fault or not (except, of course, when the mutation causes compile time errors)?