Even without cameras I am pretty sure (from my past reading on ADAS systems) that you can infer driver attention from steering wheel angle position.

If you analyze patterns in wheel angle and rate of change of angle over time, I would suspect that drvers who are texting/'looking down a screen' are going to have very distinct series of 'abrupt' correction patterns.

Where that telemetry is available in cars post 2010's then it should absolutely be used in forensics.