If it is not authentic, the autographs were most likely printed with the rest of the photo. Look at one of the autographs through a magnifying glass. If you see a dot pattern it is a preprint. But even if you don't see a pattern, it might still be a preprint. If the photo was signed with a pen, there might be a slight indention. If the autograph was signed with a pen or marker it might look "raised." If it is a preprint, you might find ones just like it with a Google image search.
Also Bonanza cast signed photos are sometimes forged. Then you have to look at genuine, known examples and compare those with yours.