spectroscopy is a good way. Nevertheless, it is a little difficult. There are several papers for measuring plant secondary metabolites by spectroscopy. Also, these two books can be useful for improve your knowledge:
1. ANNUAL PLANT REVIEWS, VOLUME 40 :Biochemistry of Plant Secondary Metabolism. Second Edition. Edited by Michael Wink.
2. Recent Trends in Biotechnology and Therapeutic Applications of Medicinal Plants. by Mohd Shahid, Anwar Shahzad Abida Malik and Aastha Sahai