Priority Group Nodes to mimic Cisco ACE backup pool. Need to alert on when we go to backup nodes

Looking in the LTM logs I see no way to alert me when a node priority group goes to the alternate nodes. I'm trying to mimic a Cisco ACE where they use backup serverfarms/pools so I want to know when I on the backup nodes. Just trying to pick it out of the LTM logs but not seeing anything that I can key on.