It is surprising to me just how many devices shipped today do. Quite why they do it I don't understand since I never see it mentioned in feature lists.

All Apple's laptops have for quite a while, however with the iPhone they still didn't have 5 Ghz on the 4s and only added it on the 5 - not that Apple let you run a WiFi analyzer on it anyway.

The Android Wifi Analyzer app shows what access points are detected and their signal strengths, channel usage, encryption etc. It is not a traffic sniffer - some screenshots at https://play.google.com/store/apps/details?id=com.farproc.wi... I couldn't find anything similar in the app store. I did find several apps that scan your local network (eg Fing).

No one mentions 5GHz in their specs. You can't tell from Google's Nexus pages which devices support it. Apple's older pages (eg Mac Mini) similarly don't say anything but newer pages (eg iPhone 5 and iPad) do. For any vendor about the only clue is they sometimes mention 802.11a support.