I would say that when evaluating a teacher, you need to know something about their knowledge base, and, if possible, ascertain their level of caring for the students. (In response to a previous post, a PGCSE may not all be a guarantor of a good knowledge base; this is not always easy to predict based on one's CV). Another very important quality of a teacher is their ability to communicate their knowledge clearly. Being pleasant and likeable could also be important, but i would say it's less important than the previous qualities I've mentioned. Student opinion can be taken into consideration, but I would say you need peer observations along with observations from a knowledgeable supervisor order to make an accurate assessment of a teacher. As an example, I remember two of my main 6th grade teachers well. One was wildly popular, yet looking back on it, he often tried too hard to be our friend, whilst the other teacher was considered bland, but I ended up learning much more from the bland teacher as his lessons were full of information and well thought out; however, this insight came to me many years later.