There are basically 4 criteria to consider:
The coache's area of expertise.
The coache's fee rate.
The ratings or "like votes" their answers get.
Their style.
Area of expertise
This is pretty straightforward. You don't hire a hockey coach to teach you golf. Unless you happen to be me and then it wouldn't matter for golf.
Fee/Rate
Consider your budget for coaching. Go with a coach who fits your style of learning, but don't spend more than you can hoping that a coach who charges more will teach you more at a faster rate. Learning and coaching just don't work that way. As well, if you're worried about the rate, you won't be focused on the learning.
Answer ratings/like votes
It's the most obvious indicator of the effectiveness of any given answer and the knowledge of a coach and his/her ability to convey that knowledge effectively.
Coach's style
There's more to a good coach than just being mega-smart and super experienced. There is also the important matter of their style or "bedside" manor. Read their answers to others, ask some of your own -- that way you'll really get an idea if they are the right coach to work with.