First of all, don't even bother with them. People are so used to seeing fake LV bags that nowadays, we ASSUME a bag is fake. If you feel the need to say something, say, "Obviously you've never owned one or you'd know that this is authentic."

Most fake LV bags have a suede-y inside, and most authentic ones have a fabric interior. If it bothers you so much that you need to prove someone wrong, just open up your purse and fish out your wallet or chapstick to give them the chance to see that the fabric inside your purse proves that it's real.