How do you know a teacher is good?

EdWonk asked about parents who request a certain teacher for their child, and got quite a number of interesting responses. It's a commons problem - in each individual's best interest to try to get the best possible teacher, but the result of everyone doing it isn't sustainable...

Anyway, among the answers it became very clear that parents consider some teachers better than others. And we teachers also recognize those among us who are stronger teachers. And kids know it, too.

My question of the day is, what tips you off? There is so much debate out there over merit pay, whether/how it could be done fairly, and lots of concern about fairly identifying good teachers, yet everyone claims they "know it when they see it," so to speak. How do you know? What do you see?

Please answer in the comments and tell whether you are speaking as a teacher, parent, student, administrator, etc. Note the subtle difference between what makes a good teacher and how you get the sense that someone is a good teacher. And keep your answers polite, please - phrase things in a positive tone ("good knowledge of subject area") rather than the negative ("not stupid").