Yeah, I would like a test device that could let me know which transfer rates a cable supports. I've got a huge pile of USBC cables (and adapters). A good chunk of them might even be USB 2.0!
It seems like a device capable of acting as a host & device could just try to shove data down a cable and see what the max is...
Take a USB3 device, plug it in with the cable, and then go to System Profiler (Apple Menu -> About this Mac -> System Profiler). Look for the device, and check the speed. If it says 480MB/s it's a USB 2 cable, if it says 5GB/s, 10GB/s or 20GB/s then it's a USB3 cable.
I assume there's a way to do the same thing on Windows and Linux.
Coconut battery shows how much power your battery is charging with. I've only used this to compare power supplies, but I don't see why it shouldn't work with cables as well. Of course that only works when the battery is empty enough to actually fast charge.
It would be helpful if the connectors for the different cables had different shapes, so it would be immediately obvious what kind of cable it is and mostly prevent plugging incompatible cables into the wrong places. Thank you for listening to my TED talk.