Can a therapist determine if your clinically depressed after one 50 minute session?

i just saw a therapist for the first time because i thought i was depressed. and after my session she determined i was "clinically depressed" after asking questions about my life. i dont have anything wrong with my life which i told her, i answered all the questions truthfully. and she concluded it could be a chemical imbalance in the brain. can someone even with the title as a therapist make a decision if they are really depressed by one session, of knowing someone for 1 hour??? i guess its not big deal, im going back for it but i think it is crazy someone can make a conclusion after that.