How do I know if the author has any authority on the subject?

The best way to find out is to research the author. You can consult online resources, or browse scientific publications for other articles by this author. In general, though, you can get a feel for the author's competence and skill just by reading the text written. If someone wrote a five-page essay on colors but spells 'black' wrong consistently, then it's not likely an author who's very competent. However, anyone is free to write what they like, as you are free to learn from it what you want. If you think the author's arguments are interesting, it doesn't matter if it's an authority in this field or not.