Can a dr. detect anal cancer through a colonoscopy?

Likely yes: As before the colonoscopy is done, a digital rectal examination is performed to visualize & palpate the area. Sometimes a biopsy of a concerning area is also needed though. See & ask your own gastroenterologist to get a better answer to this question as to what they saw during the exam.

Sometimes: Anal cancer is best diagnosed on physical exam or anosocopy since the area of concern is limited to a defined length from the anus. If there is suspicion of anal cancer prompting the colonoscopy, the gastroenterologist must be able to retroflex the scope in order to look back at the anal canal. This can be difficult.