http://answers.yahoo.com/question/in...3150943AA6FYiX
I'm not sure about NBA but for NFL jerseys, authentic ones would say "on field", the materials are thicker, and the football logo (of the player) is multi-colored instead of just the red that they used to have. Usually if the price is 40%+ off 9/10 times it's fake. If it retails for $200 and the seller is selling it for $150+ then it's most likely authentic. Of course always buy with a CC so you have some recourse.
PS: you should compare the size of the player's numbers too. Fake ones usually have same size numbers on front and back or they're much smaller than the real ones.