I can guess with a very high probability that a driver in front of me is on their phone just by the way they move in their lane. I hate driving behind distracted drivers -- they tend to do things suddenly (stop, steer, or crash).
I get so sick of hearing people say "It's okay: my car lets me talk hands free!" It isn't just the hands that are a problem ... it's your brain being disengaged that's the problem. No hands-free phone fixes that.
The bottom line is, no one should be using their phone and driving. If you have to use the phone, pull over.
Usually holding the phone up in their hand, but sometimes with the phone in a holder on the windshield.