I got a lot of questions on how to tell real Longchamp from the fake. Unfortunately, in many thrift stores and on ebay the numbers of fakes being sold for the price of real is just scary. First, you are buying a lower quality bag but paying a premium for it. The sellers often post pictures of real bags but send fakes or outright lie and say they got the item as a gift or only have one. Many of these are selling the same bag again and again (just one at a time) and have positive feedback so it is hard to tell.
The above are all pictures of a real longchamp bag in a cherry red color inside an authorized retailer. One of the distinct things is the feel of the leather. The leather itself is high quality. Another thing is all new longchamps have a white interior now.
The stitching should be perfect.
Take a look at the tags (both interior and exterior).
The fakes are alarmingly similar except the colors are often the wrong shade, and the tags have fake color codes. Other errors include wrong shade for the interior and low grade leather.