There are a couple things to study up on. One is brain wave frequencies which represent, alpha, delta, etc states .. they get a lot of hype in the woo woo circles and much is, imho, crap. Deep end. However, behind the misuse of terminology and so on there is science. This is a decent description: https://www.researchgate.net/figure/Brain-waves-charts-description_fig1_325701712. The other is brain wave entrainment. A bit of info here: https://en.wikipedia.org/wiki/Brainwave_entrainment.
The implication is that sufficiently powerful infrasound can force the brain waves to synchronize to its frequencies and thus more or less force the "subject" into a particular mental state .. fear, agitation, relaxation, etc. There seem to be medical ethical issues with any deliberate further testing so there aren't absolute results to draw on, only bits and pieces which to me seem to fit together.
There was nothing in the white papers I read about getting sick though it is in the bigfoot literature. A wild, speculative guess is that could be a result of too much power and not precise enough frequency control.
Anyway, as to your question about "so how do you know" .. I don't. Not really. I don't have equipment to detect or measure infrasound. However, what I experienced under Devil's Lake brings to mind theta state, essentially a waking trance. It's the best explanation I can find for being in what should be highly stressful situations, aware of them, yet absolutely disconnected from the emotional impacts those situations should have had.
I'm open to other ideas but they have to have some science behind them, not just new age gack or ignorant dismissal. SOMETHING happened. I want to understand it. What I've read about the frequencies of brain states and brain wave entrainment seem to be the best fit I've found so far.