According to this Windows IT Pro blog post, "Narrator – Narrator and other assistive technologies can now detect the location of the dedicated FN key, and determine if it is locked or unlocked." However, neither I nor Copilot can find documentation on how this is done.
Surface devices support this capability: both the Windows Narrator and JAWS screen readers report the state of Fn-Lock on Surface devices. I would like to know how other tools can access this information.