The cable can report what it "thinks" it is, and in fact, modern USB-C cables do this: they have "e-Marker chips" inside the plugs which communicate with whatever they're plugged into and enumerate their belief as to their capabilities. The thing is, manufacturers can set the e-Marker chips to spew lies, or a cable that used to support 80Gbps got slightly damaged after 6 months of use and now only reliably transmits 10Gbps.

Power capacity is relatively easy to measure ad-hoc via voltage drop from one end to the other...USB-PD controllers already do this and can even fine-tune the voltage to make sure that if the device receiving (sinking) power needs 20V they'll send 20.4V or 20.9V to compensate for voltage drop so that the charging device gets 20V on its end.

But actual maximum data throughput is hard to know. The only way to really "know" how much data can flow through a cable is with an expensive oscilloscope or cable tester. Because 80Gbps cables run at ~13GHz so, at minimum you need a 26GHz scope (Nyquist–Shannon sampling theorem) or more practically a 52GHz scope. And it turns out it's really expensive to measure electrical signals 52 billion times per second. The necessary devices start at $15,000 (cable signal integrity tester) [0] on the very low end and only work for max 10Gbps USB 3.2 cables, or past $270,000 for 80Gbps USB4 cables (proper 60GHz oscilloscope) [1].

On the high end, each signal integrity test device can actually cost $1-2 million [2] where the base unit starts at $670,000 plus then spending additional money for hardware-accelerated analysis, specialized active probes, and the specific PAM-3 / USB4 compliance software packages.

0: https://www.totalphase.com/products/advanced-cable-tester-v2...

1: https://www.edn.com/12-bit-oscilloscope-operates-up-to-65-gh...

2: https://www.eevblog.com/forum/testgear/uxr1104a-infiniium-ux...

The USB Situation