For anyone (in the future) wanting to find the command to check that airtime fairness is running in their system, this command should return an output (ath10k and ath9k):
cat /sys/kernel/debug/ieee80211/phy0/netdev\:wlan0/stations/*/airtime
cat /sys/kernel/debug/ieee80211/phy1/netdev\:wlan1/stations/*/airtime