Normally, if it has the JanSport logo on it, and also the JanSport tag, then it is probably authentic. If you're order offline, then I suggest going to the JanSport website. You never know what...
obviously, check the label. also, check the quality: are the seams even and tightly sewn? the leather real? the hardware clean and untarnished? also, is the style of the bag actually a chanel style?...