When I was in college, during one class that I was a student in, the instructor got on the subject of psychosis (which is when someone is out of touch with reality).
I asked her that if a person claims that he/she is being stalked by someone or that the government is after them, how do shrinks know if what the individual is saying is true or not?
She said therapists and psychiatrists can usually tell because if someone really does have one or more persons that's going after them, they'll usually tell you the specifics. But if nobody is after them, even though the person believes otherwise, they usually won't tell you the specifics. Instead, they'll say "The government is after me, but I can't tell you anymore than that because if I do tell you, they'll kill you too!"
So yes, their are ways that shrinks can tell if someone is very paranoid and out of touch with reality, or if the person really is in danger.