> Therefore, the 3rd molecule detected in the brain tissues of our subjects, hygrine (an alkaloid present in the leaves of Erythroxylum spp. only), was essential to determine that the molecules detected in these human remains derived from the chewing of coca leaves or from leaves brewed as a tea, consistent with the historical period.
If I'm reading this right, they checked for a number of markers and one of those is found only in coca leaves.
[0] https://www.sciencedirect.com/science/article/pii/S030544032...