Unfortunately, there isn't an easy way to do this with GTM sync groups. About the only thing you can do is check log messages to see which devices have joined/left the sync group. Messages would appear in /var/log/gtm and look like this:
gtmd[11895]: 011a5008:1: SNMP_TRAP: BIG-IP GTM /Common/ () joined sync group default
And you can see if synchronizations are occurring by checking /var/log/ltm and looking for messages like this one:
Mar 20 18:37:05 hostname notice mcpd[6608]: 010714a0:5: Sync of device group /Common/gtm to commit id 263 5992942257632424507 /Common/hostname 0 from device complete.