Windows 10 (anniversary update): just go to WiFi connection properties (Settings ⇒ Netowrk & Internet ⇒ click on the SSID name, look for properties).
Windows 7-10:
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netsh wlan show interfaces |
Look for «Channel», 1-14 are 2.4Ghz, >34 are 5Ghz.
Linux:
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iwconfig |
and check «Frequency».
On router side, Mikrotik routers (winbox, web):
Wireless ⇒ Registration ⇒ «Interface» column (check interface mapping on «Interfaces» tab)