For example, if I plug a USB 3.0 device and find that the I/O performance is rather slow, how do I check whether it's because the device is actually doing USB 2.0 (even though I'm using a USB 3.0 cable, connected to a USB 3.0 port on my MacBook Pro).
Try menu > About this Mac > System Report...
Select USB on the left, then find your device in the list.
The reported speed ought to be a giveaway. Mine shows it's only running at USB2 speeds [but that's because I don't have a USB3 machine]
List of speeds & common names…
USB 1.0 / Low-Speed: 1.5 Megabits per second (Mbps)
USB 1.1 / Full-Speed: 12 Mbps.
USB 2.0 / Hi-Speed: 480 Mbps.
USB 3.0 / SuperSpeed: 5 Gbps.
USB 3.1 / SuperSpeed: 10 Gbps.