It's usually obvious pretty quickly. First one student exclaims "Yes!!!" when he sees me. Then this sentiment is repeated as others come into the room. When I mention that their teacher might be sick, the class cheers. And then they start to tell me about all the things that their teacher does that annoys them.
Sometimes I can see why they don't like their teacher. I may have met him/her. I may have seen enough in the lesson plans to get the sense of it. But sometimes the anger is a mystery. Whatever. All that matters is that I have been thrown into the midst of this battle, and I need to keep from becoming a casualty.
Today I covered one of those teacher's classes. I dutifully tried to get through the lesson plan, but most of the students did not have books (they were supposed to check them out of the library), and they fought me about the story. It was "boring" or "confusing". But they wouldn't let me explain things. (I get so tired of talking over their conversations because they don't hear me anyway.)
Teens can be so judgemental. It's amazing that they like anything at all.