I think the only way is to check for FC-VI adapter. You may have sync mirror without metro cluster; and you may have metro cluster without sync mirror (yes, you should not do it, but technically it is possible).
Second best is to check for normal disks on FC switches, but that leaves out stretch clusters and V-Series.
Well, yeah, I will get the root volume name via aggr-get-root-name and than check raid_status of the aggregate where root volume resides. It's quite general now, but I've been expecting some more elegant approach :).