I always have guys who are clearly bouncing the bar off their chest say that they arent. Including a guy on this forum who posted a vid of a massive 200+ lbs. bench I think he clearly bounced.
Now short of forcing said individual to enter a pl meet for example and get three annoying red ligths, I thought of a test to see if they are controlling the weight and not bouncing it.
Lay a pencil on your chest perpedicular to the bar and lower the bar to the pencil and then lift the bar. I think that that would clearly show said knuckle head that he is bouncing the weight of his chest. Because if you can really control the weight you could do it without breaking the pencil.
If your really good I supppose you could use a piece of pasta.