My rule of thumb in supermarkets is to look at origin and choose an extra-virgin olive oil that is clearly identified as Greek/Cretan. Tesco has an own-brand Greek oil that is cheap and good, Carrefour sells something similar in France.
My prejudice is to avoid anything that says "made from EU olives" or "mix of EU and non-EU". Avoid Italy, as it seems to sell a lot more oil than it produces olives, especially these days with the blight in their olive trees. Spain had some awful scandals (admittedly many years ago). Greeks are serious, almost religious about their olive oil, and they produce a lot of it, so I believe there is less funny business. What they don't consume domestically or sell under their own brands goes to Italy for blending.
