That is why you discuss the implementation in-person and ask questions about the design decisions...

As if people who interview for Google don't just look up the most common interview questions and memorize answers before the interview...

Still I think that would become obvious pretty quickly. I've actually had people start writing out a textbook algorithm that just solved the wrong problem. And then completely stumbled when trying to explain how the program would arrive at the intended result.

I usually ask questions until the person is out of his comfort zone, and if he is completely clueless on how to proceed at that point it's a red flag.