Optical rotation is the property of all essential oils to bend  polarized light off a straight line by a certain number of degrees. Any additions that are optically active may alter the overall optical rotation of an oil.
Refractive index is the amount by which the oil deflects normal light from a straight path, in a similar way to a prism, and this also changes if the ratio of compounds alters.

Gas chromatography and mass spectrophotometry machines identify types and percentages of components in an essential oil. Together, the two machines can be used to determine the exact nature and composition either  natural essential oil or aromachemicals.