"Who is it by?"
Although "Who is it by?" would be accepted and understand by any English speaker, I don't think it is technically correct. The question should be "It is by whom?"
"Who is it by?" ends in a preposition. The answer to the question would be, "It is by him/her." Since the answer to the question is "him/ her," the question should be "whom." If the answer to a question is "he/ her," then it is appropriate to use "who."