Looking at the "hostapd_cli -i wlan1 all_sta" output reveals some possible indicators.
I tested with my Android10 phone: when I forced the AP to WPA3 only, the phone's data in hostapd_cli output showed a field sae_group=19, which is not present on WPA2-connected devices. Similarly there is AKMSuiteSelector=00-0f-ac-8, which should indicate SAE.
Phone as seen with AP set as WPA3 only:
root@OpenWrt:~# hostapd_cli -i wlan1 all_sta
ac:57:....
flags=[AUTH][ASSOC][AUTHORIZED][SHORT_PREAMBLE][WMM][MFP][HT]
aid=1
capability=0x431
listen_interval=1
supported_rates=82 84 8b 96 0c 12 18 24 30 48 60 6c
timeout_next=NULLFUNC POLL
dot11RSNAStatsSTAAddress=ac:57:...
dot11RSNAStatsVersion=1
dot11RSNAStatsSelectedPairwiseCipher=00-0f-ac-4
dot11RSNAStatsTKIPLocalMICFailures=0
dot11RSNAStatsTKIPRemoteMICFailures=0
wpa=2
AKMSuiteSelector=00-0f-ac-8
hostapdWPAPTKState=11
hostapdWPAPTKGroupState=0
rx_packets=170
tx_packets=97
rx_bytes=29675
tx_bytes=45692
inactive_msec=2370
signal=-49
rx_rate_info=10
tx_rate_info=722 mcs 7 shortGI
ht_mcs_bitmask=ff000000000000000000
connected_time=9
sae_group=19
supp_op_classes=51515354737475767778797a7b7c7d7e7f808182
ht_caps_info=0x012d
ext_capab=04000002
Same phone when AP is in mixed WPA2/WPA3 mode, likely the phone has a WPA2 connection:
root@OpenWrt:~# hostapd_cli -i wlan1 all_sta
ac:57:...
flags=[AUTH][ASSOC][AUTHORIZED][SHORT_PREAMBLE][WMM][MFP][HT]
aid=3
capability=0x431
listen_interval=1
supported_rates=82 84 8b 96 0c 12 18 24 30 48 60 6c
timeout_next=NULLFUNC POLL
dot11RSNAStatsSTAAddress=ac:57:...
dot11RSNAStatsVersion=1
dot11RSNAStatsSelectedPairwiseCipher=00-0f-ac-4
dot11RSNAStatsTKIPLocalMICFailures=0
dot11RSNAStatsTKIPRemoteMICFailures=0
wpa=2
AKMSuiteSelector=00-0f-ac-6
hostapdWPAPTKState=11
hostapdWPAPTKGroupState=0
rx_packets=98
tx_packets=41
rx_bytes=9413
tx_bytes=6573
inactive_msec=20640
signal=-51
rx_rate_info=10
tx_rate_info=289 mcs 3 shortGI
ht_mcs_bitmask=ff000000000000000000
connected_time=32
supp_op_classes=51515354737475767778797a7b7c7d7e7f808182
ht_caps_info=0x012d
ext_capab=04000002
(Interestingly, the phone seems to prefer WPA2 in the mixed mode???? )