I have confirmed that the default settings on the AirPort Extreme does set up both bands as two distinct frequencies.
.....and assign the same wireless network name to each band. Optionally, you can assign different names to the 2.4 GHz band and 5 GHz band
I can also see that the devices that support the higher frequency seem to choose that frequency by default
.....if they are close enough to the AirPort.
but I can only guess this by the type and rate listed in the Wireless Clients list on the router.
......no guessing should be needed if you look at the PHY Mode in Wireless Clients. If a device connects using "ac", then ac will be displayed as part of the connection. For example, my iPhone displays as 802.11a/n/ac.
Example of my iPhone display in AirPort Utility:
A 2.4 GHz only device displays as 802.11b/g/n. Keep in mind that 5 GHz can also be an "n" connection, even though a device is not "ac" capable. Example of a 2.4 GHz only device:
But, I don't have a PC here, so I can't check AirPort Utility using AirPort Utility for Windows. And.....I don't think "ac" even existed when AirPort Utility for Windows was offered 10+ years ago, and Apple never updated the utility from that time, so you might see 802.11n, but I doubt that you will see 802.11ac in the Windows version of the utility.
I glance at Wireless Clients from time to time, but the only thing that I really much attention to is the connection quality........"excellent", "good", "fair", "poor". The only time that I check this, is when a device seems to be slower than normal.
Although, my WiFi test from my PC, which allows me to select the preferred WiFi band does indicate that the router shows an AC connection when 5 GHz is selected.
As I said before, 5 GHz can also be an "n" connection.....it is not automatically or necessarily an "ac" connection. A device that is capable of connecting to "n only" at 5 GHz will still display as 802.11n in AirPort Utility. It won't display as an "ac" connection, and it won't display as an "802.11b/g/n" connection.
Also, interesting is that despite being right next to the router, 2.4 GHz band produces a 6 Mbps download speed and 5 GHz gets about 30 Mbps, whereas my iPhone gets about 100 Mbps on 5 GHz.
Interesting because I have a security device that connects at 802.11b/gn, which is 2.4 GHz....(it cannot connect at 5 GHz), and it will connect to the network at 130 Mbps. But, I can't test the Internet download speed on this device because there is no means to do so.
It sure would be nice if there was a free utility (somewhere out there) that would give me a list of 2.4 GHz clients and 5 GHz clients.
If you can post back with your list of Wireless Clients and the PHY Mode for each, I can probably tell you whether each device is connected at 2.4 GHz or 5 GHz.
Otherwise, maybe another user is familiar with a PC Application that will provide detailed information on WiFi connections over a network. Good luck!