Ooooooh fistulas, ouch, really not nice, been there meself hun so sympathy and empathy if you do have one.
If you're not sure if they would or wouldn't specifically look for fistulation, then I suggest, if you go for colonoscopy, just tell them what you suspect, to give them the 'heads up' (as they say these days lol)
I 'did a Google' for you and found this bit of info on one of the Crohns sites, hope you find it informative. and good luck
Diagnosis and Treatment of Fistulas
External fistulas can be found during a physical examination. Internal fistulas can be seen during an upper or lower GI series, CT scan, or colonoscopy.
Colonoscopy is used to look inside the rectum and entire colon for signs of fistulas. Colonoscopy is performed at a hospital or outpatient center by a gastroenterologist or a radiologist. The health care provider will provide written bowel prep instructions to follow at home before the test. The person may be asked to follow a clear liquid diet for 1 to 3 days before the test. Laxatives and enemas may be used before the test.