To diagnose an anal fistula, doctors physically examine the anal area and look for an opening on the skin, states Cleveland Clinic. Doctors diagnose vaginal fistulas using pelvic exams, dye tests and cystoscopy to look for damage inside the urethra and bladder, notes Mayo Clinic.
A doctor may use an anoscope to view the anus and rectum interiors if an anal fistula is not apparent on the skin. Once the doctor sees the fistula, the next step is to evaluate the direction and depth of the fistula tract, notes Cleveland Clinic. Examining the anal area using ultrasound or magnetic resonance imaging helps doctors to define a fistula tract more accurately.