This might be totally stupid but I just thought of this in the middle of the night and wanted to jot it down before I forgot.
If you point a video recording device at a mirror frozen near absolute zero and flash random numbers next to the camera, keeping track of the sequence of random numbers as you go separately, if indeed we are in the Hugh Everett Universe, we should notice upon playback a discrepancy of the random numbers reflected back and those that are recorded in our sequence.
The reason for this would be the mirror would not have decohered and would be in many states at once, essentially a window to all possible occurances of the experiment, thus having different random numbers being shined on its surface just maybe reflecting back. Maybe you would just see white light then as another possibility? I don't have a clue but would be interesting to find out.