are you looking to buy an LV? have you visited the "my poupette" site yet? (link is below) when i was doing my research a few years ago (i wanted to buy a used LV on ebay) this site was tremendously helpful. i was looking only at one specific style, the cabas piano, so i could go on and on about logo alignment and leather placement on that one only. i no longer even have the bag and i'm NO expert, but this site will help you get started spotting fakes!
This only works if you've had it for a while, but the leather handles turn a darker color instead of the same tan color. Although maybe they are making fakes with real leather handles now...
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Bad taste is like a nice dash of paprika. We all could use more of it. It's no taste I'm against. -Diana Vreeland
The symmetry of the bag will often help. The LV design is normally centered across the face of the bag. The "LV" part is never cut in half on a seam. Also, if there are straps that touch the face of the bag (I'm thinking of my cabas piano here), the design is perfectly symmetrical around the straps-- so the design on the left side of the left strap is the same as the design on the right side of the right strap.