Managed Fortiswitch are not accessible through PING, SNMP or other tecnologies. They are fully managed by the Fortigate Firewall. But it should be possible to interrogate the status of the switches through the FW byself (similare like the Fortigate AP).

There is no other way to monitor the Managed Fortiswitch

https://docs.fortinet.com/document/fortigate/6.4.0/new-features/87594/snmp-queries-to-the-fortigate-switch-controller-for-fortiswitch-and-port-information-6-4-2

We have snmp mibs and snmpwalks here