The bottom bracket is removable and should be done so for inspection. The biggest giveaway, however would be the drastic change in weight. No biker of worth would have a bike that weighed more than 15-16 lbs. The lighter the better, which is why carbon fibre is so popular.
Actually, that is not quite true, at least not for the Tour de France. Building a bike that is 10 lb or less is not a problem. However, bikes entered into the Tour de France are weighed and if they weigh less than 15 lb, ballast is added to bring them up to 15 lb since that is the minimum weight criterion that they have established. So... let's say a bike was built that was ridiculously light... say 5 lb. That would mean that since ballast is going to have to be added anyway, a cheater would stuff 10 lb of battery into the frame since weight will have to be added anyway. http://www.forbes.com/sites/allenstjohn/2015/07/25/how-tour-de-france-weight-regulations-will-make-your-next-bicycle-better/#41477d7626b4