You can use stmfadm to list the number of active sessions and the initiator IQNs when using iSCSI or FC in target mode.
#> stmfadm list-target -v Target: iqn.2010-08.org.illumos:02:7c333f92-41c9-4bca-f462-b6fd38cbf281 Operational Status: Online Provider Name : iscsit Alias : Default Protocol : iSCSI Sessions : 1 Initiator: iqn.1991-05.com.microsoft:clutch.casa.local Alias: - Logged in since: Fri Apr 24 12:10:45 2015