It’s tricky knowing the olive oil you’re buying is high-quality, fresh extra virgin olive oil. In most U.S. stores, I have found olive oil with harvest dates on perhaps one out of 20 bottles. Some have “sell-by” dates, which are usually two years after harvest, though there are no standards for a sell-by date, so there is no guarantee how old your olive oil is unless there is a harvest date. Look for a harvest date within the past year.
Even if it has a harvest date, you still won’t know whether it has been harvested and handled to maximize polyphenol content.
The way I handle this is by going to a specialty Italian shop or somewhere that I know sells California or Texas olive oil, making sure the container is opaque and has a harvest date, keeping it in a cool, dark cabinet at home, and using it up quickly. I save expensive olive oil for drizzling on salads and use canola oil for cooking, especially with high heat.