How to tell if my MacBook Air supports USB 3.0

USB 3.0 often has blue lining inside the port.

If you want to know specifically for your macbook air, go to apple support page http://support.apple.com/specs/ - Click on "Browse by Product" and enter your Macbook Air serial number. It will show you the Tech Spec specifically for your Macbook Air series, and the tech spec will tell you what USB you have.

You were looking in the right place, it does tell you there. Take a look at the highlighted part in my screenshot below.

A USB Hi-Speed bus is USB 2.0, A USB 'SuperSpeed' or USB '3.0 Hi-Speed' bus are both USB3.

Tap the black apple, then choose "About this Mac, followed by Support, then Specifications. Lots of info there, including number and type of USB port