How do you know if your doctor is good?

In a word, trust.  If you don't trust yours, change doctors.  It's your health, and that's serious.  You want a doctor who will "really" care abut you and not treat you like a number.  You want a doctor that can help manage your diabetes well and can answer your questions and communicate effectively with you. 

Instead of having a general doctor manage your diabetes, you may want to have your general doctor refer you to a diabetes specialist (i.e., an endocrinologist).  An endocrinologist receives specialized diabetes education, and focuses on the treatment of diabetes, as well as other glandular diseases.  They are specially trained to help manage your blood glucose levels better than a general doctor can do.  

We were lucky and were referred to a great retina specialist (read about my loved one's experiences as a diabetes patient undergoing vitrectomy surgery to avoid blindness), but our experiences with some of her general doctors added unnecessary stress at a time we both didn't need it.

From dropping her as a patient due to insurance reasons, to being slow in getting pre-op forms to the hospital for surgery, to not taking adequate time to fully explain which medications to take and how to administer them properly, we had to find a different general doctor--and quick.

We would highly recommend any diabetes patient to do likewise.  It is YOUR health that is at risk, not the doctor's or health care provider's health.

Ask family and friends if they are cared for by any good doctors with diabetes knowledge and education as a first step, then as a second step, there are several online resources that can be found and you might be able to find a site that reviews doctors located in your area and particular specialty needed, such as an endocrinologist.

When you do an internet search, simply enter the specialty (e.g., "endocrinologist") and if you have the doctor or specialist's name, enter their "full name" and "specialty"; city; state; and "doctor review," and you might get at least one or two online doctor reviews for your particular area.