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If you don't know what you're looking for, you don't have someone you trust to help you, and you're unable to sample each oil in a taste test, the best way to buy high quality Italian olive oil is to look for the D.O.P. (in Italian; P.D.O. in English) certification symbol on the bottle [1, 2]. There's a similar looking symbol if the produce is also organic. This symbol means that inspectors have physically gone over the entire operation of the producers and checked it against a list of standards.

Other things to look for: the bottle says if it's filtered or unfiltered (the unfiltered stuff has much more of a peppery kick), there is a production date, there is a best before date, it says that the oil has been cold-pressed and mechanically extracted, and there is an indication of the region of Italy that the olives come from.

I have never been disappointed by using this method, but of course not every good oil is certified. For reference, today I bought Planeta DOP Val di Mazara oil, which is even available on Amazon [3]. Their price is 50% more expensive than what I paid though.

[1] http://italian.about.com/b/2008/07/19/waiter-my-cheese-has-b...

[2] https://en.wikipedia.org/wiki/Geographical_indications_and_t...

[3] http://www.amazon.com/Planeta-Extra-Virgin-Olive-Oil/dp/B005...