When they talk, do they engage the attention of the class, present them with comprehensible input and also allow them to interrupt, comment, ask for clarification, and so on?' 'Is the teacher checking on comprehension as she or he talks?' 'If so, what kinds of comprehension-checks are they using?'
I am not saying that teachers should always talk, that good teaching consists only of talking interactively with the class or individual students. I think that students learn not only through 'comprehensible input' but also their own output. But I don't believe at all that a 'good lesson' is one in which students do all or even most of the talking. Some lessons may be good if they are carefully structured in such a way that students do a good deal of the talking and at the same time get a lot of feedback, both formally and informally, from the teacher about their performance. But this is by no means true of all lessons.