Best UX method to evaluate a game controller?

I am developing a hardware controller that controls a game using biometric data without any interface on the PC side.

If the device is effective (I'm not sure how to measure this, thus asking the question.) it will have a calming effect on the user when it lowers media tempo and volume in response to heart rate changes. Outside of calm mode, this gauging would be similar but I am trying to determine the best user testing method to gauge how well it calms the user. The challenge is how to get the user into a stressed, non calm state to begin with while still being willing to conduct the test. (to simulate the normal use of the end product)

Since the UI is the biometrically controller game controller itself, what is a good method to evaluate the interface with multiple users?

I would like to gauge how effective the device is without simply asking the user "do you think it's working?"

here is the link to explain how it works:

https://hackaday.io/project/159030-optimote-bio-adaptive-computer-interface