1. Ensure that the font of your OPI NAIL LACQUER has the right font and is it aligned properly and not skewed.
2. The real OPI brush has an embossed logo on the top of the brush. You can try using tissue to dap it so as to see it.
3. The original OPI has the same serial code engraved on the top of the bottle, bottom of the bottle, painted in black and same code number on the sticker at the bottom of the polish bottle.
4. There is a peel off sign, and when you peel it, there is a list of ingredients. And on the other part of the sticker, the color name and code is printed. And you can never stick back the 'peel here' sticker.
5. There are 2 steel balls inside the polish bottle. More/less = fake!
6. The inside of the cap must be jagged, not smooth.
7. Many are saying that the warning symbols at the back must be 4 otherwise it is fake. But OPI has addressed the issue: "The number of symbols on bottles may vary due to regulation changes and does not mean that a bottle is fake if there are only 3 labels".
8. There is always the address of the OPI distribution center on the back of the nail polish. This address will be in the Netherlands.
I've found them for many sources online.
Pictures from Here and my own.
Hope this helps :)