How can you tell a USB cable version?

The USB-IF says all fully compliant USB 1.1 cables sold also meet the specifications for USB 2.0 Hi-Speed (although low quality, non-compliant cables may not work at the higher speed).

USB 3.0 cables include nine pins (instead of just four for USB 2.0) and have a larger "B" (device) end that will not fit into USB 2.0 printers, scanners, etc. These cables seem to often have a blue color and/or an "SS" (SuperSpeed) marking next to the USB logo.

I don't think there's any difference between USB 1, 1.1 and 2 but version 3 has extra pins near the back as shown here. I'm not sure how obvious it would be just from looking at the cable since they're hidden at the back of the plug.

This table from wikipedia shows how you can use the color of the plastic inside the connector to tell different versions apart

USB 1.x             White
USB 2.0             Black, sometimes white
USB 3.0             Blue
Sleep-and-charge    Yellow or red