[Tristan Lall]

Short answer is you can't prove it. Not rigourously, anyway.

There are all sorts of pretty obvious ways to cheat, but you're expected to resist the temptation.

Instead of proving it, your team will have to present a signed log form indicating the circumstances of the openings and closings of the bag. There's no harm in taking lots of pictures—because that can only help you if something unforeseen happens—but FIRST is for the most part willing to take your team's word for it.