WhatCable

Tells you what a USB-C cable and port are actually negotiating, rather than what the packaging claimed.

What it does

WhatCable reads the USB and Thunderbolt topology and reports what the connection actually is: link speed, whether a device negotiated USB 2.0 instead of 3.x or 4, port and hub layout, and power delivery details — from the menu bar.

USB-C hides all of this behind an identical connector. A cable that charges fine may be USB 2.0 only, which turns an external SSD into something ten times slower than it should be with no error anywhere. This makes that visible in seconds.

Notes

Available from homebrew-cask but developed on GitHub by an independent author; it self-updates, so the recorded version lags. system_profiler SPUSBDataType gives the same underlying data on the command line, considerably less legibly.

Alternative to

Alternative	Type	Trade-off
System Information → USB / Thunderbolt	Built in	The same data, badly presented
`system_profiler SPUSBDataType`	Built in	The command-line version of that panel
A labelled cable drawer	—	Unfashionable, and it prevents the problem outright