[an_ko]

I do this by reading the code's logic aloud, and while doing so, noting the things that I have to remember to understand it. This includes things like "what was the name of that variable", "what does this function do", "has the parser read a comma", etc. Complex code results in larger "mental stacks", simple code smaller ones.

This isn't really "proof", but in my experience, such a demonstration is easy to understand and leads to constructive conversations and good results.