Most counterfeiters, especially sellers in China and those who import "wholesale" from China will mislead you, with stock photos or photos of authentic bags. This can be a trick!Always double-check your bag after you receive from the seller! Check the details of the bag once you have it in your hands to make sure that what you received, is the real thing. The advice stated here is entirely my own personal opinions. Happy shopping!
Other BagsFrom what I've seen on eBay and at places selling knock-offs, the Pliage style totes seem to be copied the most. This includes purses, tote bags, cosmetic pouches, and coin purses. Please notice, this guide below does not apply to authenticating bags outside the regular Les Pliages line.
PliageThese tips below apply to the Les Pliages line of folding nylon canvas bags (not to Planetes, limited edition Pliage bags or the leather bags). I have here pictures of two bags. The blue medium with long handles one is real, and the pink and tan shown are fake. Sorry photos are blurry....
GENERAL
The fake is on the left, the real deal on the right. Right away, notice the left bag looks too shiny and the handles don't look right: too short for long handles, and too long for short-handle variant.
Before buying, eyeball the dimensions, check the measurements, then compare with official Longchamp website. Longchamp new colours of the canvas bags 2 times a year (for fall-winter and the spring-summer) but the leather trimming is same always.
Leather
There is contrast stitching on the leather parts of the bag, which is a different colour than the leather itself on the visible side (and same colour as the leather on the back of the flap). Many Pliages I have seen have plastic tubes in the handles, because the leather is soft and not very durable. Real Pliages have no plastic tubes and are made from durable, high-quality leather.
The fake leather looks like a fish-scale pattern, with clean lines of even depth.Sometimes it looks smooth or bumpy. On authentic bag, it is almost a diamond pattern, and grain is not embossed as deeply as the logo.
Leather Flap 
An authentic bag has a jockey logo embossed on both the leather AND the snap. If there is no logo at all..... beware!!!
OLD LOGO: (shown above) There is a jockey on a horse impressed down into the leather, and a straight line underneath. On the knock-off, the logo is not centered above the snap, the lines of the impression are not as clean as on the real, and the horse is lacking details like hooves.
NEW LOGO: (not pictured): Longchamp has introduced a new logo on the leather flap,though on my most recent trip to the shop they had the same bags with different versions of the logos. The new logo has the jockey and horse embossed, so the logo is raised up above the leather. There is _no_ line underneath, and if you look at the back side of the flap there will be a mark where the logo is raised.
The bag above is real. The back of the flap has writing above the stitching. This may, however, vary from bag to bag. On my "shopping" totes the words modèle déposé and made in france are between the lines of stitches (see arrow). Longchamp, Pliages and the name of the bag are above.
Among real bags, there are variants. On a backpack I saw, the writing is all above the seams. Some say Pliages, some say Le Pliage.. Longchamp does not make all bags in France. Some Pliage are made in China.
Model Name
On my fake, the words modèle déposé are missing the accent marks. In any case, I think you have a fake for sure if the name printed on the back flap does not agree with actual model or it just says the word "type" but not _which_ type:
sac a dos - backpack
type L - large tote, short handles
type M - medium-large tote, short handles
type S - small tote
"shopping" - large tote, long handles; medium tote long handles
cabas - rectangular, no zipper
Reinforced Snaps
An easy one! Real bags have their snaps reinforced with clear plastic discs that extend beyond snaps on the inside of the bag. The foldable Pliages also have snaps near the bottom.
Fabric and Lining
Bags should have crisp seams, and perhaps creases from the original folding. Your bag should fold nicely and the two parts of the snap meet easily. The fabric on fakes I saw is too shiny and texture is much smoother compared with real. If a bag looks too shiny in a photo (see top photo!), you may want to double-check it's other features.
Inside, the plastic coating that lines the bag feels like plastic but not rubbery. For pastel bags (light blue, pink, so on), lining should be WHITE, not same color as outside. The tan fake above is lined in tan, when it should be white. Dark colors are lined with black or brown.
Zipper Pull 
Longchamp Pliages have round logo zipper pulls, which can be gold or pewter depending on what shape the bag is. The real one is extremely detailed, and on the fake the bodies of the jockey and the horse blend together. Longchamp currently use YKK zippers, and you can see the letters on the zipper above.
Backside of Flap
Fake bag leather looks hairy and the edges jagged. Real one is also raw but is smooth and clean. On the snap itself, the fake has the words "guang tong" and real says, "ORIGINAL.
** Thanks to Ebay for publishing useful information for Longchamp Lover. You can click here.
