From what I have learned, it varies by specialty. All practicing physicians are required to fulfill a certain amount of CME (continuing medical education) credits every year, and take licensing exams on a regular basis (which varies by state and/or specialty). Being "board certified" is a little different (for example, surgeons who are licensed in a given state can practice but do not have to be "board certified", just "board eligible") and is done every so many years as determined by the board of that specialty. I don't know if certain specialties require all practicing that given specialty to also be board certified or not...
Most residents take a test every year (gen surg--ABSITE, ortho--OITE, etc.) in their given specialty to allow programs to assess the resident education and identify weaknesses in their program. Some programs may require a certain score to be attained to allow a resident to advance to the next year, but from what I have seen only a few programs OFFICIALLY do this.