I think you have a couple options:
You can leave the APs configured for DHCP but assign DHCP reservations on the FGT. That way, if the APs ever rebooted, they could be guaranteed to get the same AP you reserved from the FGT.
Not sure if you have a FortiAnalyzer or a syslog server, but you could look for the "ap-leave" (or "ap-fail") action from those logs. I have not been able to confirm this but I'm pretty sure that log will tell you if the AP is no longer communicating with the FortiGate.
Edit: Confirmed the logs.
I do have a fotianalyzer and I'm probably vastly under-utilizing it! I will investigate, thank you for the quick response!
No problem. Look into using to creating an event monitor. What you are looking to do should be pretty straight forward to accomplish.
Ping back if you have questions.
I was finally able to set some time aside today to work on this. Works great, thanks again!