I was curious what people were buying back when this thread started, and a lot of people posted about Lacoste Tennis shirts and getting fakes.
The easy way to tell is that Lacoste uses mother of pearl for the buttons. Mother of pearl doesn't retain heat like plastic does, so if you feel it on a sensitive part of your body (like your upper lip) it will feel cold to the touch. What's odd is that I just compared the one Lacoste tennis shirt I have to my RL Polo shirts and the Lacoste was cold, the 2 Chinese made RLs were room temp, and a Peruvian made RL appears to have mother of pearl buttons in addition to a much better quality weave of the cloth and possibly better cotton as well. Knew the quality seemed different in the past, but never inspected the labels or noticed the button material between the different RL Polos. Kinda cool!
Anyway, if a knockoff is going to use mother of pearl buttons, then who cares if it's a knockoff, the thing is going to be higher quality than most shirts anyway. If they're using the fancy buttons they probably are woven and put together well too.
EDIT: With that said, don't spend much money on polo shirts. I just grabbed a couple new ones from Lands End for $20 without a label on the outside. Polos are essentially "t-shirts for grownups". If you're going out in public and just want to throw something on for comfort, use a polo. If you're meeting people for anything social after say 5pm, then wear something else IMO. This, sadly, is something I have only very recently grasped as I transition into preppy douche. Along with undershirts, jeans, and underwear, this is something you can skimp on. Spend the money you save on shoes and a watch. And if you save a lot of money, have your **** tailored.