Does anyone know how fabrics, etc are tested for NIR compliance?

Seems like the whole process of making a textile NIR compliant and testing for it is hush hush.

I think it's "hush hush" because it's more pseudo-science than real science.

The NIR spectrum is just light...in a wavelength humans can't see without assistance. In the visible spectrum, you want camo that reflects light similar to how your surroundings reflect light (i.e., if you're in a bright empty desert, you want light tan colored clothes that reflect the same wavelength and amount of light as the surrounding sand...wearing black that doesn't reflect at all makes you stand out). The same applies to hiding from devices looking in the NIR spectrum...if you reflect too much you're bright white, and if you reflect too little you're solid black.

I did a bunch of experiments years ago with various pieces of gear and treatments in front of a cheap webcam (setup a MOLLE panel with a lot of different pouches). Spray paints tend to make things non-reflective (black). Fabrics that are vat dyed (not printed) tend to have some slight variations in reflectivity and retain their pattern, but not always...rothco's woodland BDU's retained their pattern in NIR but most of the multicam gear I had was an almost solid highly-reflective blob. Rite dye didn't have any noticeable effect.