Symptoms are most often what prompt people to seek medical attention that leads to an anal cancer diagnosis, but sometimes the cancer is found through a routine digital rectal exam or an anal Pap test. If anal cancer is suspected, then further evaluation is required to confirm the presence or absence of cancer.The first step in diagnosing cancer is to view the anus and rectum more closely. This is done endoscopically, with a hollow, lighted tube called "anoscope." The anoscope is lubricated and inserted into the anus, allowing the doctor to get a better view of the anus. The procedure is called an "anoscopy" and is brief and painless.
If a suspicious area is found during an anoscopy, then the doctor may remove it or do a biopsy. A biopsy involves removing small samples of abnormal tissue to be later viewed under a microscope by a pathologist.
The pathologist studies the biopsy and looks for cancerous cells. If cancer is confirmed, then he or she will also determine what stage the anal cancer has progressed to. Staging refers to how far the cancer has spread to nearby tissues. For those with suspected cases of advanced anal cancer, more tests may be done to determine the anal cancer stage. Tests may include MRIs, CT scans or other imaging tests.
source : cancer.about.com
