How do you know if a Franco Sarto bag is authentic?

One possible answer is: it depends on where you bought it. If you got it on a street from a vendor who offered you a really cheap price, chances are it's a fake. If you bought it from a reputable department store (like Macy's or J.C. Penney's), chances are it's authentic. As for online, the same rules apply. If you got it on eBay from a seller you don't know who offered an amazing bargain, be suspicious-- I am not saying the seller is dishonest, but people who offer an amazingly cheap price may be offering merchandise that is not authentic. Instead, by from reliable stores or websites that guarantee their merchandise and you should be fine.