Due to the increasing popularity of aromatherapy, you can find many essential oils on the market. But there is nothing mandatory informing the customer about the quality of the oil contained in the bottle.
You can rely on the seller's reputation, and the prices displayed. According to their different production costs, differences in pricing can be huge, from one oil to another.
The word "oil" does not necessarily mean that an essential oil has an oily consistency. On the contrary, for most species, the pure product is of a rather watery appearance. Normally, if you pour a drop of essential oil on a paper towel, it must evaporate and leave no trace. An oily stain may be a sign that the product is not pure, but cut with another vegetable oil, for more quantity at a lesser cost.
Experts will rely primarily on the smell of the finished product. With years of experience, they are able to distinguish the good products from the bad ones. Some prefer this method to the gas chromatography analyses performed in the laboratory.