Best way to prove where people have been, although you'd like to think you could rely on honesty, would be to make use of those camera things everyone carries around.

Everyone would have to upload a selection of their photos from the day into a new set on flickr, but it's invalid if it doesn't include full EXIF data for public view. A lot of cameras actually record GPS coords of where photos were taken too. Make a point of taking a shot of something from where you are. This could be anything, e.g. a bridge, a pub, a monument, etc. Just a suggestion like.