You ask a great question. It highlights the difference between board certification and experience performing a particular procedure, in this case, Juvederm filler injections.
The American Board of Medical Specialties grants board certification in the broad specialties of Plastic Surgery, Dermatology, Otolaryngology-Head & Neck Surgery, Ophthalmology, etc. You can confirm your doctor's board certification by contacting the ABMS at www.ABMS.org. This will confirm that your doctor met the educational requirements, but it says nothing about thier experience doing a procedure.
There is no reputable organization that certifies doctors as experienced or qualified at doing individual procedures. Picking a good HA injection doctor is like picking a good facelift doctor. You start with education and board certification, and then seek recommendation from trusted physicians and nurses. Next you visit the doctor's practice and have a consultation where you discuss your concerns and how he or she would address them. If a surgery is involved, you doctor may have credentials to do the surgery at a quality hospital.
Looking at the doctor's photographs can also be helpful. You may visit several before you make your decision. Best of luck.