Code coverage tools are available. Porting software to other platforms is just one crude, but very effective way of testing different code paths, some you can't even see (unless you are in the habit of stopping at each compile stage and checking things such as the compiler's assembler output . . . ).

> BSD doesn't support a bunch of stuff. How would you deal with that concretely. How would you ensure that all those different code paths are tested?

You could start by explaining "a bunch of stuff" -- and why it's necessary -- concretely.