Plant quality, harvest and production technique has a lot to do with quality. Environmental conditions also directly effect the percentages of each component of the essential oil. Botanical variety, chemotype identification and origin are also a part of quality determination. Most all Elizabeth Van Buren essential oils are organicly grown or wild crafted. Pesticides and anti fungal agents used on foods are routinely analyzed but it is not practical to analyze for them in essential oils as to the kind and quantity of these chemical agents.
Analysis is the only really objective way to know.
An essential oil does not have to be adulterated to be inferior. Of course, environmental conditions directly affect the percentages of each component of the essential oil. Just because an essential oil is grown organic does not mean it is verifiable or unadulterated when you buy it. The essential oil should not be adulterated. Lasty, the essential oil should be labeled natured, tested, organic, wild crafted, etc.