How can I determine if an iscsi device is a mounted Linux filesystem?

Try iscsiadm -m session -P 3 and check the last section "Attached SCSI devices"

Just FYI: If you set iscsid to start on boot you do not need to unmount/logout iSCSI partitions prior to reboot. As long as the partition is in /etc/fstab and has a _netdev option.