When I was in Rome, a fellow I met told me that, in Italy in general but Rome in particular, you should NEVER look while crossing the street. If the drivers see you looking, they'll know you've seen them and they won't stop. He said you should just step out into traffic without the slightest hint that you might have noticed them. Only then will they stop.
My guess is that then they understand that not only you are aware of them, but that if you survive, you will possibly be able to identify them.
It goes without saying that I would only do that on pedestrian crossings.