If you consult a doctor for anal fistula, the doctor asks your medical history and examines you physically. Some of them can be easily spotted while others might need closer examination. The doctor might use his finger to examine the fluid and oozing in that region. Better consult a rectum and colon specialist for such problems as you might have to go through X-Ray and colonoscopy.
Colonoscopy is the process where a doctor looks inside the bowels with the help of a tube with a camera attached at the end. This helps the doctor to diagnose the anal fistula.