USB 2.0 and USB 3.0, how can you tell the difference?

USB3 has more pins than USB2.

USB1 and USB2 have four pins, two for power, one pair for data. USB3 has those four in the standard place, and two extra pairs for 'superspeed' communication.

Wikipedia has two pictures of this, though I must admit is is not clear when you look at them. It should be much easier when looking at a physical device. (Note that both USB3 ports are blue in the pictures below, which matches @mdpc answer.)

Added the picture from Scott's link here:

On the hardware side, I have been noticing that the plastic in the plug is generally blue for USB 3.0 and white for USB 2.0.

I also believe there is a special USB “symbol” that denotes a USB 3.x connector.

On macOS, System Information indicates the USB speed, per this answer on askdifferent

For reference, USB speeds/specs are as follows:

USB 1.x - 12Mb/sec
USB 2.0 - 480Mb/sec
USB 3.0 - 5Gb/s
USB 3.1 - 10Gb/s

On Linux, lsusb per this answer on askubuntu:

...and I haven't confirmed it, but apparently this is the lsusb equivalent with Powershell:
gwmi Win32_USBControllerDevice