I worked at Barnes and Noble for 5 or so years, so I can let you in on the secret.

Whenever you read a book, you leave a kind of psychic residue behind, the natural result of your brain’s electrical signals leaking out through your optical nerves. Barnes and Noble employees are the only living humans trained in the deep forensic techniques required to detect this residue.

Just kidding. You have your receipt? Take a look at the book you want to return. As long as there aren’t any obvious marks, you’re okay. I’d worry more about whether you’re within the time limit (it will say on the back of the receipt) but I remember managers being flexible on this as well.