0: https://www.usb.org/sites/default/files/USB%20Type-C%20Spec%...
Speaking of which, every time I ask the question about “tell me just from looking at a USB C cord…”, I learn something new.
Plugging it in would probably be a good start. I can't name a single time where I've ever mistakenly identified a USB-C cable. Sometimes I've plugged it in when it wasn't connected to the wall, but I think worrying about non-compliant USB-C cables is no more relevant than caring about non-MFI certified Lightning cables.
99% of people only care about power (and optionally data), so that's what the spec focuses on.
Regardless, the fact that DP-Alt-Mode exists is not a refutation of the base USB-C spec. You're either spec compliant or you're not; same as Lightning without the license fee.