Rule 1: Don't just look at the price. With olive oil, you get what you pay for. Cheap olive oil tastes cheap.
Rule 2: Do trust your own instincts and eyes, rather than brand names and advertising. The only reason to look at the label is to ensure you are buying 100% extra virgin olive oil, which has had the least processing and tastes best.
Rule 3: The most attractive color usually indicates the best quality olive oil.