[qznc]

You need formal machine-checked verification (Coq, Isabelle).

Unfortunately, this is not enough. You need a consistent and complete specification, too.

[skew]

The first half is a bit easier - if you can show some implementation meets the specification it is at least consistent.

The second isn't so clear, but sometimes you can get a bit more confidence by showing that other good properties are implied by the specification alone.