The big distinction is that a false confession manga can show actions and expressions directly through illustrations. A novel has to paint those pictures in the reader's mind with words. And sometimes, the complexity of the plot and character development can differ based on the author's style and the limitations of each form.
Another way is to compare it with officially released versions. If there are significant differences in the story, characters, or overall presentation, it could be a fake manga.
The 52nd chapter of the manga "Confession to a Beautiful Master at the Beginning" was about his master being poisoned by the green lotus. The specific plot and plot could only be understood by reading the manga.
It's possible for manga box sets to be fake. Sometimes, poor-quality reproductions are made and sold as the real thing. This can be difficult to spot for the average buyer.