let’s say someone just read about particular symptoms online and now they think they have that disorder when in reality they do not. since they are actually believing that they have this issue they will act like they have the issue.
how does pdoc will possibly know that the person doesn’t have that disease in reality? there are not brain test like we have for blood.


Your premise is wrong. Just because you belief something doesn’t make it real. For how it can be figured out: there are brain tests, it’s called psychometrics. These tests are specifically designed to be resistant to “faking it”. For some diagnoses, they also use data such as third party accounts, teachers notes, etc.
does it also work on minor disorders like AVPD, Anxiety etc?
Yes. The point of the diagnostic protocol is determine what issue it is and the severity.
Shit anxiety and depression is so easy you can do a 1-10 fill in the blanks about common situations and how you feel. Just remember lying about it doesn’t help anybody.
Unless you need an anxiety diagnosis for a medical marijuana card.