Basically, I think the most important items to consider in assessing an essential oil are three: the smell, the price and the bottle.
In relation to the aroma, obviously the oil used in aromatherapy should smell like the plant part which is removed. For example, if we uncovered a vial of jasmine oil, and smell it, we must clearly perceive the scent given off by the jasmine flower (well known by most people.) If we can not quickly associate the smell of jasmine, the flower, then we are faced with an adulterated oil.
The assessment we conducted through the price we use the essential oil in aromatherapy is very simple, but has nothing to do with the misconception of “at higher prices, higher quality.” In this case we are going to consider is whether all oils have the same value or if the type of oil, its cost varies. For example, we entered a nudist business and ask the seller the price of essential oil of rose, he looks at us smiling and indicates a value, then ask for the essential oil of fennel and we answered the same value, and so on until the seller tells us that all oils are equal. Why should we be wary of the quality of these oils if that happens to us?
The bottle of essential oil that we buy should always be dark (usually brown or blue), this is because the oils are very volatile and evaporate easily on contact with light. Remember here also that a very nice bottle or decorated, does not imply that the content quality is good.