While I cring at the thought of referring to a half empty roll as used TP, I would hope that most people could tell the difference between clean TP and not clean. If the roll has been used by a previous guest and there is no evidence of prior guest placing the soiled paper back on the roll, why not simply do as several other posters have suggested and tear off a few sheets and go on about your business. I'd much rather see the time hotels spend replacing TP rolls be spent cleaning the previous guests hairs off the bathroom floor. But that's another topic altogether.