msm-notifier will tell you
msm will tell you
mhwd-kernel will tell you
the packages themselves will sorta tell you because deps will start breaking and the kernel will stop getting updates and existing in the repos, etc.
Normally you will see a warning message about a kernel being EOL while you’re updating the system, and I believe that the Manjaro Settings Manager notification icon in the system tray will also warn you about that.
Other than that, the Announcements threads also always mention it when a kernel goes EOL.