It's merely a declaration. There's no way to prove it apart from the declaration, which we take in good faith. The return may not materialize, but the point is that at the time of making the application, a person must have the intention to return.
We keep information about those Canadians, and when they're getting closer to the five-year point, or the “anniversary date” as we call it, we ask them whether they've returned to Canada at some point to reside here, which ensures that they are not unnecessarily removed from the international register. If they don't respond or have not returned to Canada, they are taken off that register after the five-year period currently stipulated in the act.