would be expensive and take time and never happen, but I think having high FPS cameras pointed at their mousepads (at least for somebody like flusha) from the ceiling and the ability to sync it up with the game would be ideal. if they do some preset 180/360 turns as control variables, you could then determine exactly how far he moved his mouse and about where he should be looking in game. No idea if this was at a lan, but this method would have determined whether or not he manually flicked there instantly.
he has a really low sensitivity, that's why people who defend him say "as he turns he needs to reset his mouse, that's why it lands on walls". that gif, and a few current gifs i've seen, have had substantial enough flicks that it should be obvious when the camera gets slowed down.
of course it won't help determine whether or not he's using a very small aim assist while tracking people, but it would clear up all of these supposed aim asissted-wallhack flicks. something like this would be noticeable considering his sensitivity.