I think that the most important aspect of picking a coach is synergy. It has to be someone you feel a connection to so you will work well together. A business coach should have previous business experience - not just book knowledge. They should be able to draw on their own experiences to help you grow your business or resolve whatever issues you are having. If sales is an area you wish to work on make sure your business coach has actual sales experience. Understanding sales from the inside is far different from reading about it or hearing about it from others.
A good business coach will act as an accountability partner and supporter of their client. You want a coach who listens well and will help you go where you want to go - not where they think you should go. Those are very different things. A good coach will help you take a real look at where you are and where you want to go. A good coach will tell you the truth - even if it isn't what you want to hear. You are paying them to help you succeed - not blow smoke up your dress.
Expect a coach to help you create expectations and goals for yourself. Sometimes the coach will be a mentor, a teacher, a cheerleader, . . . The coaches job is as I said - to help you solve whatever is getting in the way of your success and develop strategies that will work for you.
I think a coach should be able to hear what you say and what you don't say and ask the right questions to help you get where you want to go.