This is a legitimate concern. Although I wouldn't say it is hard.

There have been several issues in Italy with the quality of olive oil made there. I'd be very skeptical of most of what is being exported by them.

Oxidation from improper storage/bottling and mixing with more refined oils reduces quality. Be sure to buy a darker bottle in an amount you'll use fairly quickly. Keep it in a cool and dark place.

Always check the harvest date on the bottle. It should preferably be from the same year but for up to 2 years is fine.

Be sure to do your homework on particular brands before making a purchase.

As far as brands go I prefer the 100% Greek Kalamata from Trader Joe's. Kirkland Toscano from Costco is good also.