Perianal adenomas are usually a benign tumor that affects older, intact, male dogs. In male dogs these occur in tissues around the rectum that are responsive to androgens (like testosterone) and develop into benign tumors under the effect of these hormones. Unfortunately, when these tumors occur in spayed female dogs they are much more likely to be malignant than not. A malignant version of this tumor is known as a perianal adenocarcinoma. There is no way I know of to tell them apart based on appearance visually. The tumor should be removed and sent to a pathologist for histopathologic examination. Careful examination of the surrounding tissues might reveal metastasis of the tumor. If your female dog is unspayed, there is a greater chance that the tumor would be benign but it would still be more likely to be malignant than in a male dog.