I would think the only way you could prove that statement would be to find someone with a genetic alteration that makes them incapable of acquiring language. Could you elaborate?
The basic evidence for language acquisition being genetically determined, in the same way learning to walk is, is that it has all the characteristics of genetically determined behaviour.
It occurs at set stages following a clear developmental order, there appears to be a cut off time, the level of skill acquired is basically the same in all members of the species, and it appears to proceed independent of widely differing types of input.
Compare that with learning a second language, or learning to play chess or play the piano.
Chomsky wrote over twenty years ago that he was amazed that linguists opposed the idea of language acquisition was genetic because they didn't consider the science hard enough, yet geneticists and evolutionary biologists, both followers of a 'harder' science, accepted it without reservation.
Finding somebody with a genetic defect that prevents him learning to speak would be as difficult as finding one that prevented him learning to walk. The hypothesis presumes a single gene responsible for either ability, and that is not the way genes work.
If you want a layman's introduction look at "The Language Instinct" or "The Way the Mind Works" by Stephen Pinker.