In a terminal, just issue:
And for one network interface you will get something like this, where "5.18GHz" indicates you're on the higher band:
wlp1s0 IEEE 802.11 ESSID:"My dual band WiFi" Mode:Managed Frequency:5.18 GHz Access Point: FF:FF:FF:FF:FF:FF Bit Rate=6 Mb/s Tx-Power=20 dBm Retry short limit:7 RTS thr:off Fragment thr:off Power Management:on Link Quality=51/70 Signal level=-59 dBm Rx invalid nwid:0 Rx invalid crypt:0 Rx invalid frag:0 Tx excessive retries:0 Invalid misc:0 Missed beacon:0
If you want to know what channel you're on you can install the
iw command e.g. with apt on Ubuntu and get something like this:
Interface wlp1s0 ifindex 3 wdev 0x1 addr ff:ff:ff:ff:ff:ff type managed wiphy 0 channel 36 (5180 MHz), width: 80 MHz, center1: 5210 MHz txpower 20.00 dBm multicast TXQ: qsz-byt qsz-pkt flows drops marks overlmt hashcol tx-bytes tx-packets 0 0 0 0 0 0 0 0 0
which indicates channel 36 in this case. But I guess that was probably obvious from the frequency anyway in the previous query?