One simple way to check whether or not your emerald has been treated is to look through a microscope to check if any blue or yellow/orange flashes from internal cracks occur. These colored lines are more likely to suggest clarity enhancement.
Note, however, that certain rainbow-like interference colors can naturally occur in #unenhanced natural emeralds. This is why this approach is not 100% reliable and you should always verify your stones with a qualified #lab if you are unsure of the results.