You can enable debug level hostapd log and check syslog for below messages.
IEEE 802.11: authentication OK (FT)
WPA: FT authentication already completed - do not start 4-way handshake
Client should start 4-way handshake only when it first associating at the AP and when steering/switching over same SSID, it should be 2-way handshake FT (802.11r fast transitioning) stating in above messages.
edit: yes you just click on enable 802.11r but when doing on different devices just make sure they have the same mobility domain, as well as same SSID and password set.