How to check status of the console cable connection?

If the BIG-IP is connected to a console server I would run a script that would connect to the BIG-IP port on the console server, send some carriage returns and see if there is any output.

If you are trying to figure out if someone is intermittently connecting a console connection to tamper with the F5 I don't know of any way to do this. Ultimately something like that would probably be easier to validate whether the person attempts to login to the BIG-IP