There is such a thing as an "inauthentic Catholic."  To know one, obviously, takes knowing what  "authentic Catholic teaching" is.  That there is such a thing as "authentic Catholic teaching" also seems obvious.  When someone teaches as Catholic that which clearly isn;t, I don;t see the harm --- in fact, I see good --- in pointing that out.  And if that person routinely spouts stuff that's not Catholic, that person should be known for what he is, an "inauthentic Catholic."