How about a government enforced mandate that everyone gets a secure computer embedded into their skull in a way that removal will kill them, with a camera that continuously records video, sound and GPS positioning? This could then digitally signed and timestamped by the individual's computer.

Then, when a questionable event happens, the video of all participants and anyone nearby can be replayed. Then, all videos of participants can be replayed to make sure they match up, and the video of the time leading up to the event could be scrutinized. Furthermore any one who visited that area in the recent past could be checked to make sure they didn't create some sort of hologram or something that the participants watched rather than the real thing.