I mean I think some books it can be very clear about an authors stance based on the material. Though I guess technically we can never be 100% sure about that, I think it’s fair to make some educated guesses that an author that writes a historical fiction book about the civil rights movement in America and writes those things in a favorable light most likely agrees with the civil rights movements beliefs.

I do think sometimes it is harder to tell the authors opinions and like you said unless you follow them on social media or know them in real life we will never 100% know for sure