It's been reported widely that there has been fairly widespread adulteration of oils sold as olive oils, either not containing any at all, or being mixed with cheaper oils.
One method of dealing with that is the hope-so, guess-so method such as "Well, this is the 'Newman's Own' brand, and it's very expensive, and Paul Newman wouldn't have swindled me, or at least I hope not, so I guess hopefully his brand wouldn't" which very well may work out fine in some instances and I suppose probably that one, but which isn't necessarily reliable.
In terms of what I could find in the supermarket that had label information giving any better confidence, previously I used the Colavita brand, as it has the Italian CERMET certification of being authentic.
While I can't read Italian, this seems like a legitimate organization: http://www.cermet.it/
The Colavita though is what I'd call mid-high priced.
Since then I've seen one of the types of Pompeian -- the Organic Extra Virgin, Natural Flavor -- has a certification from an organization called NOAA.
This also seems legit: http://naooa.mytradeassociation.org/sealprogram/announcement.shtml
The Pompeian I would call mid-low priced. So that is what I use now.
So here are two ways that appear reliable for having confidence of olive oil being authentic.