Looking at certain sites just becomes a muscle reflex. The second I'm bored, I feel the urge to look at my phone.

If you have a large enough rotation of websites, you never feel properly bored, so you can spend the entire day mindlessly browsing.

I think it's easy to tell whether you're browsing with intent or just killing time. It's just hard to close the lid and go do something else, especially if you've been doing it for so long that you forgot what "something else" is.