The good way of knowing is to look at the assembly generated by the compiler. Maybe not a completely satisfying answer given that the result is heavily vendor-dependent, but unless the high-level language exposes some way of explicitly controlling it, then assembly is what you got.

> The reason people do potentially more expensive mix/lerps is because while it might cost a tiny overhead, they are scared of making it a branch.

This is a problem, though. People shouldn't do things potentially, they should look at the actual code that is generated and executed.