Yes, people seriously counterfeit nail polish. If you're buying your OPI from a reputable store like Ulta or Trade Secret then obviously you have nothing to worry about in terms of authenticity. But many of us, myself included, sometimes turn to Ebay or Amazon to track down older OPI polishes or get a cheaper price. Aside from doing your research into a seller's feedback history and using common sense (if it's too good to be true, it probably is), there are a few tricks you can use to differentiate the fakes from the real deal. Here are some details authentic OPIs will have.
There are a couple little details to look out for on the front of the bottle. "mL" should be lower-case m followed by a capital L and "Fl" should be capital F followed by lower-case l.
The details on the back of the bottle will vary depending on what era your OPI came from. OPI collections from around the 2009-2012 era (The bottle you see below is from the Holland Collection, released in spring 2012.) will have three warning labels in this order:
Slightly older OPIs (but still with the green label on the bottom, not the black) will have four warning labels in this order:
I'm not sure exactly when OPI made the switch from four to three labels but for reference the polish above is A Good Man-darin Is Hard To Find from the 2008 Hong Kong Collection so I'd say somewhere around 2009 is when they originally dropped the fourth label.
Anything from the Skyfall Collection (Holiday 2012) onward now carries four warning labels again, in this order:
Older OPIs (the ones with black text on the bottom label rather than green text) will have three warning labels. Any black label polishes manufactured from around 1997 to 2002 will have only two warning labels, not three. Any pre-1996 polishes will simply have warning text in place of labels. EDIT: A thank you goes to reader Ashley for helping me fill in some of the date-specific info about older OPIs. I don't have any OPIs with zero or two warning labels in my stash, but the ones with three will have the labels in this order.
On the bottom of the bottle, there will be two labels on top of each other with a "PEEL HERE" tab. The top label should have a barcode, color name and color code. The text will either be green or black depending on when the polish was originally made. Make sure the color code (the five characters listed underneath the color name) and name match what is listed on OPI's official site.
The second label has the color name and code. Any polish that has only one label on the bottom is suspect. The second label underneath should look like this:
Newer OPIs (say, collections that came out around 2013) have similar-looking second labels, except that the company name is written horizontally above the color name, rather than wrapped around the edge of the sticker as you see in the picture above.
However, please note that OPI minis do not have labels or names on the bottoms but the color code should still be inked onto the bottom of the bottle for reference purposes. EDIT: Thank you to reader FabbyNailedIt for sharing that minis will also not have any mixing balls like the full-sized bottles do.
The majority of OPI bottles will have engraved numbers somewhere on the bottle. There should either be black numbers stamped along the bottom or white numbers faintly embossed across the top--or both. The numbers are unique to each bottle so it's just a matter of whether or not they're there rather than what exactly the numbers are.
However, some of the Japanese-exclusive OPIs, older polishes or ones that come in duo sets will not have engravings. For instance, my Serena Williams duo polishes did not come with any engravings on them. A lot of my newer OPIs from 2013 also do not seem to have these engravings, so perhaps it is a feature that's being phased out. EDIT: Reader Fluff-Anna from Sweden has also reported that her authentic, store-bought OPIs do not have the white engraved numbers, only the black stamped numbers, so it's possible that the numbers may vary from region to region.
Some retailers (even legitimate brick-and-mortar stores) will file the white numbers off the top of the bottle, which according to OPI they aren't supposed to do, but if you notice a little dent around the top of your bottle, that's likely what happened.
Fake OPIs are also known to have crooked or visibly shorter lids than genuine OPIs. Many fakes also have a visibly rougher texture on the top of the caps than genuine OPIs do. There should also be a small dot indentation on the "P" on the cap.
The inside of the cap should have ridges like a gear. EDIT: Thanks to Rachel for pointing out that DS polishes do NOT have gears inside the lid since they have silver tops, not the usual black ones. Black label OPIs also do not have the gears inside the caps.
The wand of the brush for genuine OPIs should have OPI imprinted on one side of the brush. You will likely have to wipe polish off the brush since it can be pretty faint. Apparently some of the fake bottles have this imprint as well, so you'll need to do some other tests to make sure you've got an authentic product. Older OPIs (pre-2006) also may not have this imprint.
I've also heard that some fake OPIs smell really terrible, since they're likely made with different chemicals from genuine OPIs, so that's something to look out for as well. Some fakes are easy to spot while others can be a bit trickier since manufacturers of fakes are becoming increasingly attentive to detail. Remember to always do your homework before purchasing and trust your gut!