First and foremost, you need to register with italki to be a teacher. If you don't do that, they will ban your account. To register, you need to send proof that you have the qualifications or experience to be a professional teacher instead of a community tutor. 

Once you're approved, you need a welcoming and friendly video, and class descriptions that attract the type of student you most want to work with. This is important because if you enjoy your work, then you'll be more successful.

Finally, after you start teaching italki students, listen to them and what they want from you. Then prepare your classes for the needs of your students. It won't always be easy, but it will be rewarding. 

1) To be a good teacher : I think following a well structure and understanding students requirement is impprtant to become a good teacher.

2) If a teacher is good : check the feedback , take a trial class. They are is really not a good way to understand if it is gonna work. It really depends on the realtion and dedication of both student and teacher.