Hi John,
Unfortunately, there’s no built-in way to immediately detect when a remote Host goes offline—there’s no continuous monitoring. The only indication is the status icon in the address book.
There is a kind of workaround—you can open a View-only connection, and if the Host goes down during the session, the Viewer icon will start flashing/blinking on the taskbar. It's not a guaranteed method, but it can serve as a basic indicator.