Which comments are racist?
Generally speaking, people are racist; comments aren't racist. Remarks can indicate a person has racist leanings or is a racist, but no single comment is, by itself, a probative indicator of one's being thus. Some remarks are more probative in that regard than are others.
At the end of the day, I doubt that many folks in the U.S. will attest to being racist; consequently, in determining whether any given individual is a racist, observers must use one's statements -- what one has said, how one said them, the circumstances in which one said them, what be the foreseeable implications of the statements, etc. -- to determine the nature and extent to which one is a racist. That said, slightly racist and overwhelmingly racist are both racist.