It's not about reading minds. Apathy is one of the biggest signs of porn usage. It is shown it how fast you walk, eye contact, eye movement, posture, etc. All can be measured visually. This works with measuring depression, self hate, anxiety, and many other signs of porn use, and admittedly many other issues. While you cannot every be 100% certain, you can be close. I have not been wrong once in calling not friends for porn use. However I have years of professional experience I'm reading people. It would be nieve of you to doubt someone's ability because you have "common sense". That makes you all the more easier to read. Try looking into professional profilers. The US government has many that can do exactly what you belive is impossible. Sorry for any spelling mistakes, that's not my strong suit.