Okay, now there are few things to look for before you bid:
1. The front.
2. The warning signs at the back.
3. The lable at the bottom of the bottle.
As you can see, 1st layer contains product name, product code, a line of black printing, barcode, and "PEEL HERE" at the bottom.
The 2nd layer has product name and code in the centre and other info all around it. No barcode on 2nd layer.
If the bottle has only either one layer, FAKE. Double layer but not exact set out like shown above, FAKE.
4. Steel balls x 2
note: see the space at the bottom of the bottle is smooth and even? Some fakes can be spotted from miles away using this test alone.
5. Engraved numbers
Most real OPI have engraved numbers on the bottle. The digits of the numbers differ from bottle to bottle. They are a little blurry and hard to read. Some bottles from Japan or sold in a kit might not have the engraving at all. Only FAKE OPI will engrave them big and clear, they still fake.
6. The brush
7. Inside the cap
8. The seal on the cap
I probably don't need a pic for this. The plastic seal with "Exclusively ProWide Brush" DOESN'T go on every color. In fact, only a handful of them actually have the seal. They are to indicate that this particular color is using Prowide Brush whereas others are using standard brush. If you see a seller showing off all his/her stock with the seal on every single color, they are FAKE.
A bottle of real OPI will pass all the tests above and gives you the true quality and fullness of the color you after. Ask seller questions and ask them to provide extra photos before you bid or buy. Grab yourself a real bargain, not a rip off!