When there are no outstanding HTTP requests, DOM events, CSS transitions or setTimeouts the page can be assumed to be rendered. Not all pages will enter this state, so some heuristics are likely used.
What you are describing is not possible with, for example, Phantom. HTTP request observation is, but AFAIK the others are not. I know it's likely that Google has something far more advanced / customized, but just wanted to point this out for anyone thinking about doing this themselves; it's a really hard problem to solve.
