Dual band WiFi is faster. It also avoids interference with Bluetooth. For example, if you're using a Bluetooth mouse, headphones or speakers, they can interfere with WiFi reception since they're both on the 2.4GHz band. The 5GHz WiFi band is thus less polluted by Bluetooth and all those 2.4GHz access points around you.
If the specs say 2.4/5GHz WiFi then it's dual band. If they don't say, it's time to check our reviews Both the XPS 13 and Acer Aspire S7 have dual band WiFi. Most (but obviously not all) machines that cost $1,000 or more are dual band.
-------------------- Lisa Gade Editor in Chief, MobileTechReview