Unfortunately, I have. It's a major pet peeve, especially if I've paid a premium for a new, pristine bag. I don't know about other people, but I can tell by appearance and the smell. It seems fairly easy to me to tell that the lining is used. Stretching and softening of the sides and bottom of the bag can be a giveaway. Areas of wear that come about from using a bag, not just what I call "closet use." It depends. I've looked at so many bags, I just can tell. I know that sounds crazy, but . . . .