670nm is 670nm, no matter the marketing and/or medical certifications & approvals
In theory. In practice without testing there's no guarantee you're getting only 670nm light. I can make a brilliant 670nm source that also douses you with 385nm light.
Not really. In practice, the vast majority of LED producers provide the Spectral Power Distribution in the datasheet.
Agreed, but I'd also argue that's what medical certs and approvals are doing in this setting.
A spectroscope is a handheld optical device which allows you to see the light distribution coming from a light source. Should be able to pick one up for under a hundred bucks.
I've actually looked into this in the past (for measuring the spectra of LED bulbs), but they're significantly more expensive than I expected! Gratings are cheap, but digital models seem to run upwards of $2k. Any suggestions for midrange models would be appreciated!
Save money and go analog! Calibrate against a known spectrum. One example would be a sodium vapour street light but there aren't as many around these days.
Guerrilla? Like you mean to jump out of tree cover and shine red light at people randomly?