Check the bottle. A quality supplier will sell their essential oils in a tightly sealed dark (usually amber) glass bottle. ...
Read the label. It should clearly state the common and the Latin name of the plant used to make the oil. ...
Verify the source.
There is a test that shows if it is a pure essential oil or, on the contrary, it is of low quality. To check, you just have to pour a drop of the oil on a piece of paper, if the oil does not leave a mark and evaporates quickly, then it is a pure essential oil