I shoulder check but I'm not able to clearly make out what the other drivers are doing unless I'm way too close to be making a lane change.

It doesn't take a whole lot of effort to notice them looking down and slightly to the right, or very hard down.

Humans are good at noticing what other people are looking at, so it doesn't take more than a moment to figure out if someone's looking at something either than the road. From there, it's a relatively safe bet if they're not looking at the road, they're looking at this phone.

I see this all the time in a car, but maybe Ive trained myself to pay attention to this considering how many drivers use their phones while driving.

Phones are very easy to spot, because it's a tiny glowing screen in an otherwise dark car.