Get a bottle that says extra virgin cold pressed. This means it is the 1st press and not heat or chemical treated.
Test the color of the oil. Hold several bottles of different brands and types up to the light, look at the neck of the bottle. If the glass on the neck of the bottle is clear the glass is not tinted. If the glass is clear, then you are seeing the true color of the olive oil. Sometimes manufacturers use tinted bottles to hide inferior oil. Good pure oil has a greenish tint. Oil with a golden tint implies it is a 2nd or 3rd press, has been heat or chemical altered or it has vegetable oil added.
For the best, most pure (as would have been natural 100 yrs ago) the bottle must state extra virgin cold pressed.