How to monitor device trust certificate expiry ?

Because the device certificate isn't from a CA I would recommend putting in scheduled refresh because you know exactly when it expires and you can just create a new one. This can be recorded and then setup a reminder or something along those lines. If this is something you insist on checking every day you can automate the checks at the following URL.

https://my.f5.com/manage/s/article/K000148955