Choosing quality olive oil starts with the label. Look for "extra virgin" (extra vergine), harvest year, origin region and producer name. Good olive oil is not anonymous.
Avoid transparent bottles — light degrades olive oil. Prefer dark glass bottles or tin containers. Price is also an indicator: quality extra virgin olive oil typically costs over €10 per liter.
If possible, taste before buying. Good olive oil is fruity, may be slightly bitter and peppery — these are quality marks, not defects.