You can see the same code on the sticker at the bottom of the bottle. There is also color code. Make sure the color code matches the name of the color. You will have to refer to OPI website for the color code.
There is a "peel here" sign on the sticker. If you peel it, there is a list of ingredients. And on the other part of the sticker, the color name and code are printed on it. There is no way you can stick back the "peel here" tab.
To prove my above points, I went to SS2 pasar malam last night to buy some fake OPI with Tzethon. The bigger bottle cost RM10 and the mini ones cost RM2. With a little bit of flirting with the pasar malam guy, we got 2 mini bottles for free.
To know if your O.P.I. is real or fake, check out the colors on their OPI Nail Studio. It is like an application where you can click on the colors to match your skin color and nail length. It is really fun to try playing with that application.