More of the common things to look for when figuring out quality may be with free acidity (oleic acid). You may consider the acidity as the level of freshness of the oil and the definitive line between smooth or dense intake. Try it out, take a teaspoon or a sip of the extra virgin olive oil laying around your cabinet and see whether it goes down smoothly leaving no bland after taste or “oily” like substance left on your tongue or mouth, or whether it does the opposite. Extra Virgin are considered to be graded with free acidity of below 0.80% or 0.8 grams per 100 grams. During the process of extracting the olives for it’s oil, there are times in between where given too much time or done wrong, increases the acidity and loses its superior characteristics (making it “denser”)
Within your own search for facts and criteria to look for, it may be interesting to note how the extra virgin olive oil was made. Whether stone pressed, machine crushed, mass produced, filtered or unfiltered, some may make a difference, some may not. How they get the olives from the trees (whether they are damaged in process), the time in between picking, processing, (once picked off, the olives starts to loses its quality) and bottling, and how they store their products (maximizing life). For best results, they go by the 24 hour time frame between picking to milling; for an even better result, a few abide under 24 hours (duly note those that do, especially within a 12 hour frame). And then the techniques utilized at the mill (separation of the olives from stems, crushing, and dividing the oil from olives) may get a bit technical and wary to understand, which I’ll hold off for another time. Understanding how some things work and are processed are especially essential to how some foods are labeled quality.