In an earlier blog post, way back in July, I posted a link to an article I wrote on Essential Oil Quality Testing. Quality testing essential oils is important to know that you are getting the 'real deal'. GC/MS stands for Gas Chromatograph/Mass Spectometry and reputable essential oil suppliers will provide you with the copy of this report if you ask. It will give you information on what an essential oil is composed of, but remember each batch of essential oils is going to vary, depending on a number of factors, such as weather and growing conditions. Reports will vary slightly.
For further information on GC/MS testing of esential oils, and an explanation of the process in greater detail, from someone in the essential oil industry, please refer to Andrea Butje at The Aromatic International Blog post - Essential Oil Industry Standards: Aromatics International Supports GC/MS Testing.