To help point you toward the right starting point, tell me a bit more about your goals:
: Raised eyebrows pulled together, widened eyes, tense lips.
is a paid product, his site offers excellent free introductory resources. What’s Free: Paul Ekman Micro Expressions Test
For tech-savvy users, there are open-source, Python-based facial expression recognizers and training datasets available on GitHub (such as those utilizing the FER2013 dataset).