In my experience, the most valuable thing in judging a professor's profile is the number of classes they have done divided by the number of students they have had. For some of the best professors I have had here, the number has been above 5. For some of the worst, the number has been between 1 and 2 (meaning a large majority of the students chose not to have a second class).

Note that this method also gives a fair chance to the newer professors who may be building a good student base but who have not been around for a long time.

I think that if a person didn't enjoy the session, they will most propably leave no feedback at all. If someone has left negative feedback, this probably means that the teacher is not simply mediocre but abhorrent :) I haven't ever seen that.

I agree that the rating system doesn't work very well. The lowerst rating I have seen is about 4.5, which is still 5 stars. This way, everyone looks the same except for new teachers who haven't had sessions yet and therefore have 0 rating. Tough luck!

I strongly agree with imk about the ratio of classes to students. Maybe that should become an official thing in teachers' profiles.

The OP is correct, of course. The rating system here is useless because of people not being objective, or being "too nice". I'm guilty of this too. I rated the first few lessons objectively, then realized I was the only one doing this, so I decided I didn't want all the teachers to think I was a jerk. Now everybody gets 5 stars from me, and I never give out unflattering comments. 

Probably the only useful thing for choosing a tutor is the video. But some good teachers have bad videos, and some have no videos. Ratio of lessons to number of students would seem to be a good indicator, but I found this not to be the case. For example, the worst teacher I ever had here had the highest lesson to student ratio. This is probably because I like informal conversation. If you like a structured beginner or grammar course, it might be useful.

So unfortunately, you are stuck with choosing someone and hoping it works out. Good luck!

I think you just have to pick a teacher and give them a try,and see if you like them.

I don't think people on italki just write flattering responses for no reason.Why would they do that?

All professional teachers here have certificates and information about their qualifications,as well as videos and reviews.Many teachers give feedback to students for free (including me) so you have plenty of ways to judge for yourself if a teacher is good or not.

That's my opinion.I don't understand why you think a student would write good reviews of a teacher,if the teacher had not been useful or effective.There would be no point.

I would strongly agree with IMK about looking at the ratio of classes to students. This indicates that a teacher is well suited to their students. What it doesn't tell you is if the teacher is suited to your needs, but I think it's easy enough to establish that by looking at a teacher's interactions on italki. Do you mostly agree with their corrections? Do you think their profile is sufficiently professional. 

I also agree that having a certificate or degree does not necessarily make a good teacher. In fact, I might even go further and say qualifications might be irrelevant in some cases. I'd also add that a teacher who has taught hundreds of classes and has a perfect 5 star rating on italki might be attracting comments and ratings that are flattering and not always representative of their skills.

To answer the original question about practising for the IELTS speaking, my approach is to have a lot of general conversations with my students, to correct their most basic errors (including pronunciation and verb tenses) and to encourage them to memorise a core set of phrasal verbs and idioms that could be used in almost every speaking exam. Remember however, the new Cambridge scoring will make this much more difficult, and students really will need to have the level they're assessed at. It might not be possible to fudge a speaking exam in future.