Thought Police and Room 101

Since I read the book the first time, I have had a few questions about how the technology, and specifically the Thought Police, work. Now that I've reread the book, I was wondering how the Thought Police know each Outer Party member's worst thing in the world. Can the telescreens pick up emotions as well as recording visual and auditory information from the Outer Party Members? What if the individual did not know what their own worst fear was? Or they encountered it somewhere without telescreens, would there still be a way to pick up on what it is? Is it possible for the Party to access the memories of the people going to Room 101?

I think that the ideas of the telescreen picking up emotion could be correct. If that was not what happens then there must be more people like o'brian that spy on each person. I believe that if this is true about the telescreen's then maybe that is a look into our future, we already have things like 'alexa' which can detect tone in your voice. So i believe that peoples room 101's are chosen by the telescreen's and the though police.