Your external monitor will check for output being returned to stdout in order to mark a pool member available.
If no data was returned, it will mark pool member down.
To manually run external monitor and check for output, you can move to /config/filestore/files_d/Common_d/external_monitor_d/ where all of your external monitor scripts are stored, check for script name (I noticed BIG-IP appends some sort of versioning to the files) and then run it as as script, passing it the required parameters (I believe defaults are server ip, L4 port and timeout)
[root@f5:Active:Standalone] common # cd /config/filestore/files_d/Common_d/external_monitor_d/
[root@f5:Active:Standalone] external_monitor_d # ./\:Common\:myexternalmonitor_320535_2 <server ip ex 10.10.10.10> <server port ex. 1813> <timeout ex. 16>
This will immediately let you know if output is seen or not, or if there are other errors -- for example I noteiced PERL interpreter changing directory when moving to BIG-IPv15