How to decide if a mobile can handle 5GHz wifi for a dual-band 2.4GHz and 5GHz router
 

Superhub 2 supports both 2.4GHz and 5GHz wifi operation independently at the same time. For example, I can let my Sony Xperia M2 use 5GHz wifi while other devices like my ZTE Blade III use 2.4GHz. I found this by trial and error using Superhub2 web interface and the Fing application for the Xperia and Blade III. The manuals of the Xperia and Blade iii did not specify if the devices could operate at 5GHz as far as I could see.

For Blade III in http://www.gsmarena.com/zte_blade_iii-4983.php WLAN section says: Wi-Fi 802.11 b/g/n, Wi-Fi Direct, Wi-Fi hotspot, DLNA
and for Sony Xperia M2 in http://www.gsmarena.com/sony_xperia_m2-6146.php WLAN section says: Wi-Fi 802.11 a/b/g/n, dual-band, Wi-Fi Direct, DLNA, Wi-Fi hotspot.
I observed that Blade III does not work at 5GHz but the Xperia does. The only difference between the two WLAN specs is the 'dual-band' phrase for Xperia, so I tentatively assumed this is the keyphrase that specifies that a device can handle dual bands. Then here is a (probably incomplete) list of dual-band devices from gsmareana:

Google Nexus
Samsung Galaxy, S3, s4 and S5
HTC one M8, HTC remix
Iphone 5s and Iphone 6
Lenovo k900, X2, Z2, Z2-Pro
LG G2, G3
Motorola Moto X, Droid-ultra
Sony Xperia T3, Z1,Z2 and Z3
Xiaomi Note 4G, Mi 3
ZTE Nubia7/6/5, Grand IIs, Grand Memo II LTE, Star 1,

Most fairly expensive devices.

I am confused by the notation Wi-Fi 802.11 b/g/n in gsmarena because it includes wifi standard 802.11n. According to Wifi standards/IEEE 802.11 - Wiki.html#802.11-2007, standard 802.11n should handle both 2.4GHz and 5GHz bands but Blade III certainly does not. So is the 'dual-band' keyphrase in gsmareana equivalent to wifi standard 802.11n or the more recent 802.11ac? or have I completely misunderstood this?

Q1. are there other notations in common use that specfy that a device can handle dual band?
or is there some other way of finding out except buying a device and try it?

Re: How to decide if a mobile can handle 5GHz wifi for a dual-band 2.4GHz and 5GHz ro
 

That is correct. Dual band means the device works in both bands. If it is not dual band, it will not work in both bands. However this is only one way of stating if the device can operate in the 5Ghz band.

That is incorrect. N is capable of operating in either the 2.4Ghz band or the 5Ghz band. It doesn't necessarily have to do both.

You have slightly misunderstood this.

'N' is a set of protocols that can operate in either band.

AC is a 5Ghz only technology (or rather, it is not allowed in the 2.4Ghz band). Theoretically it could operate at 10Ghz or 60Ghz, but of the common consumer bands, 5Ghz is the only one it can use.

A/B/G/N means it can handle dual-band.

A is 5Ghz only, B is 2.4Ghz only. If it does both A and B then it does both bands, i.e. it is dual band. Similarly, AC is 5Ghz only, and G is 2.4Ghz only.

---------- Post added at 19:13 ---------- Previous post was at 19:09 ----------

Basically if it supports 5Ghz it will have either 'A' or 'AC' listed in the supported standards.

If it supports 2.4Ghz it will have either 'B' or 'G' listed in the supported standards.

Having 'N' in the list tells you nothing about supported bands.

Also bear in mind having 'N' doesn't mean it can actually do ... N. Most older 'N' phones will connect to an 'N' access point, but will not connect much faster than 'G' speeds because they lack all the additional functionality of 'N' that actually increases speeds.