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More often than not, when you pull a bottle of “extra-virgin olive oil” off a grocery-store shelf, what you’re getting is not, in fact, extra virgin. This is a problem that extends far beyond the label: impostor oils and those past their expiration dates lack the health-boosting elements found in the real deal, leaving health-conscious consumers drenched in grease.

“It basically becomes a liquid fat,” says , executive director of the California Olive Oil Council.

Mislabeled and fraudulent oils are rampant. In recent studies, some of which were popularized by , the UC Davis Olive Center found that were not extra virgin, including those made by popular brands like Carapelli, Bertolli and Colavita. (California-made oils fared much better, with nine of 10 samples authentic.) Of 15 “extra-virgin” oils sold to restaurants, meanwhile, .

Extra-virgin olive oil refers to oil that comes from fresh olives that were milled within 24 hours of their harvest, and it can’t have been treated chemically. In other words, extra-virgin oil is a fresh fruit juice. And like other types of fruits, olives contain a vast array of polyphenols, or natural antioxidants, that protect the plant during its lifetime. The older the plant, the more each one contains. That’s part of why both olive trees and grape vineyards, which can be hundreds of years old, create such antioxidant-rich products, says , a nutritionist and associate professor of medicine who researches olive oil at Brown University. “Extra-virgin olive oil is a medicine, more than a food, because of these antioxidants. Even blueberries don’t have the array, the number, that you see in [extra-virgin] olive oil,” she says.

An oil sold as extra-virgin cannot be refined. Oils most often failed because they were adulterated with a cheaper, refined oil, oxidized by age or too much light or heat exposure, or they contained olives that had been damaged or rotten from the start.

All of these issues can give olive oil flavor defects—like rancidity or mustiness—that not only legally take away its extra-virgin status, but also make it taste completely different than extra-virgin oils. Even worse, these issues also undercut the oil’s health benefits, particularly when it comes to antioxidants.

One polyphenol only found in extra-virgin olive oil, oleocanthol, is a potent anti-inflammatory shown to help prevent and fight diseases like arthritis and Alzheimer’s. You can taste it: It’s what causes the burning sensation at the back of your throat. Another compound, alpha-Tocopherol, is a form of vitamin E that studies have linked to the prevention of every ailment from glaucoma to Parkinson’s disease. Extra-virgin olive oil has the highest content of alpha-Tocopherol of any kind of oil. Oil that isn’t extra-virgin has none. 

One problem with faux-extra virgin oil is oxidization. If olives are left on the ground too long, or if more than two years pass between when the olives were pressed and when the oil is consumed, oxidization breaks down those antioxidants. “It’s like you learned in chemistry class: if you put two chemicals near one another and don’t agitate them at all, they just sit there. But if you add light or a catalyst, they’re much more likely to interact, and that interaction changes both,” says , a nutritionist and the founding director of Yale University’s Prevention Research Center. (That said, experts agree that you can cook with olive oil without undoing those properties; the breakdown only occurs when the oil hits the smoke point, which is around 405 degrees Fahrenheit for extra-virgin oils.)

Even a true extra-virgin olive oil loses its benefit two years after the harvest. “What was once a fresh, extra-virgin olive oil with lots of health benefits and antioxidants degrades,” says . But old oil is still made up primarily of monounsaturated fat, which is to claim that increasing intake, whether of extra-virgin or not, can decrease your coronary heart disease risk. “Even if you’re consuming olive oil that’s not rich with antioxidants because it’s not extra-virgin, there’s still likely to be some benefit,” Katz says. In other words, it’s still a better choice than, say, Crisco.

For those who want the full health benefits, there are some simple steps to take when buying oil.

Consider the Price

Although spending $50 is no guarantee that what you’re getting is extra-virgin, spending less than about $10 for a 500mL bottle is usually a guarantee that it’s not. You can find authentic extra-virgins for $15 or so, says Nicholas Coleman, chief oleologist at New York City’s Eataly.

Examine the Bottle

If it is made of glass, it should be dark, which protects the liquid from oxidizing sunlight; it should never, ever be plastic.

Look for a Harvest Date

Without that date, which should be on the bottle, you can’t know if the oil is fresh. The lack of a date also can mean that the oil was blended with leftover oil taken from different years. Since oil loses its best health benefits two years after the harvest, the date should ideally be the most recent harvest.

Determine Where It Came From

Look for the specification of not only the country, but also the town or region the oil came from, along with the specific olive cultivar. Bonus points for a , which is an EU-regulated guarantee of origin. “It’s like buying a bottle of wine. If there’s no year, no region, and no grape varietals on it, and it just says ‘red wine’, what do you think you’re getting? You have no idea. You can’t make an educated purchase,” Coleman says. Flynn goes a step further, saying that unless you’re buying from a dealer who has been thoroughly checked out by a third party, like the on investigative journalist Tom Mueller’s website , you should buy olive oils exclusively from California, which has the most stringent regulations.

Taste It

The real test, though, is in the taste. Although it takes practice, attributes to look for when tasting are peppery, spicy, bitter notes. That’s not just a necessary attribute of extra-virgin, but the actual flavor of those health-boosting polyphenols. Unsurprisingly, these notes will be strongest in olives that have the highest concentration of polyphenols, like , Coratina, Lucca, and Pendolino olives.