The first place to start looking is by asking your primary care physician for recommendations based on the results of their existing patients.

Call the people you have in your personal address book of friends and family and ask if they know someone who could recommend a good dermatologist, or ask them if they know someone who had acne and now it is all gone. Chances are they came across a good doctor.
Once you have some leads consider goggling their names to find reviews, read their credentials, you can bet that a good dermatologist has tons of customer testimonials.

Check their office space does it look like business as usual? Or does it look like they’ve made it personable,  do they have  successful before and after pictures of his patients or thank you letters placed any where around his office, or do they just have posters of medications. pharmaceutical companies pay them a large amount of money to recommend their products, so a lot of them have their own personal agenda as a top priority. Why would they recommend a simple effect treatment if a higher ticketed item will yield them a higher return.
How many years have they been in practice? and a good question to ask is why they choose the dermatology field in the first place.
In case of an emergency due to a reaction of a treatment how can you reach them during after hours emergency?

While a dermatologist is a considerably important resource and guide a quick look into alternative medicine and eastern medical practices such as chinese medicine acne treatment many of their practices have been developed and improved for many years. So it can also be another source in the battle against acne.