Wouldnt a GI doc be able to tell the difference in anal cancer and hemorrhoids?

Yes, they would. Yes, absolutely, i think GI docs are able to tell the difference between anal cancer and hemmorhoids. If they think something looks suspicious- like a cancer, and not just hemorrhoids- they would do a biopsy which is going to confirm the presence of particular disease.
Generally Yes. Some times it may require a biopsy and tissue examination by a pathologist.