1. On most Lacoste polo's the size is in numbers, not letters. The numbers varies from 2-8. BUT this is not the case with all Lacoste polo's. There is a lot of genuine Lacoste polo's labelled with letters instead of numbers. These are often seen on the Limited Editions and the more expensive series like the Silver line. Do NOT base the authenticity on this statement alone! If you're buying older clothes from Lacoste (1980-1990) theres a good chance these are original, the design has just changed a lot in all these years. Vintage Lacoste may also have the crodidile logo in other colors, than the green one, again this does not mean that they are fakes!
2. Look at the quality. Original Lacoste polo's are of a high quality, without any bad stiches etc. The crododile logo should be flawless too. 95% of the genuine Lacoste polo's have the Davanley washing guide inside the shirt. Most Lacoste polo's have 2 mother of pearl buttoms on the chest, but NOT all. The "new vintage" lines have 3 buttoms on the chest. If you buy a striped Lacoste polo, make sure that the spripes on the arms are in the same height in both arms. Fake ones usaully don't fit in the pattern. If you're buying a polo, that the seller claims to be a Limited Edition, go check that it really excist on the official Lacoste website.
3. Box, tags and pricelabels. Lacoste polo's do not come in a fine gift box like Louis Vuitton and other high-end brands. If there's a big gift box and a lot of extra tags etc, its probably a fake. Lacoste polo's does not have any kind og serial number or "control card" Don't look for these kinds of labels. Most Lacoste tags are white with the Lacoste logo in the middle in green. Again a few Limited Editions might have silver colored tags (like the Tom Dixon Techno polo's)