How do you know if one singing instructor is better than other? This can be difficult to compare but for the most part the process is simple. You'll find that any singing instructor can be good but learn more about their education and what makes them qualified to be an instructor.
Working with a singing instructor is hard work. While they should be firm with you and should demand results from you to keep you motivated and pushing forward, you should feel comfortable with them. You should be able to ask questions and feel good about what you are accomplishing.