Two strategies I use when I’m not sure a book is worth finishing.

1. I skim the rest of the book: First and last chapters, bullet points, scan for key words. If the results aren’t promising at least I gleaned the gist of what remained.
2. I don’t quit, I simply start reading something else. If I never go back, I figure it wasn’t compelling enough to warrant my time. If I pick it up again and still don’t want to finish, I drop it. Sometimes I find I simply needed a break and finish. If I don’t even remember it’s waiting for me, so much the better.