With regard to good/bad/fake olive oil the best way to know is to do a taste test. I'm sure you've tasted enough by now to recognize certain attributes, if not, buy an assortment at the grocery and compare. Not all extra virgin olive oil taste the same, just as in wine. And not every customer is going to like the same flavor profiles. What makes olive oil unique, as well as delicious, is the differences in intensity, pepperiness, aroma, fruitness, time of harvest, country of origin, etc, You can buy a $2 bottle of Merlot or a $200 Merlot - both are still Merlot. I suggest being more directed with educating your customers about the nuances of different varietals, which to use with what foods (again just as with wine - White is better with lighter flavor foods, red with more robust etc) certain olive oil varietals enhance certain foods in different ways. A simple way to experience for yourself is with mashed potatoes. I do this with my family and friends that think "one olive oil fits all". Make small batches with 3 different evoo's, then let them try and taste the difference. It works pretty much with all foods. I love to have a whole artichoke appetizer with 3 different olive oils for dipping, they really get it and it opens the door to the world of olive oil.