How can anybody differentiates a virus or spy-ware from an nonhazardous key-gen?

Answer 1

This is a difficult task.

This can only be done by either "trying it out and seeing what it does" or decompiling it, and attempting to determine what the program is coded to do.

In general, programs that are involved in illegal or shady activities are more likely to be infected. This is not a story told to scare people into buying the legitimate software, but a verifiable fact.