What exactly are you looking for here? The easiest way to test fail over is to just power down the active unit during a maintenance window. As for verifying if your units are ready for a test it depends on which version of F5 LTM you are running. With version 11 code you need to make sure your device groups and certificates are configured correct. In a basic setup the standby unit should be in "ONLINE(Standby)" and "In Sync" status. In more advanced configurations running multiple device/traffic groups both units might show up as "Online Active" so you will need to look at your traffic groups to see which f5 is active for each group so you know which ones will be failing when one is powered off.