Yeah agreed on the USB spec. I have a CaberQU USB-C cable tester via Kickstarter, which I can use to see what pins are connected and from there at least guess what they support.

Unfortunately the CaberQU site is down right now, but the Kickstart page is still up: https://www.kickstarter.com/projects/electr/c2c-caberqu-usb-...

I'd love to see something similar for DisplayPort, HDMI and more.