Detect if the active keyboard on macOS has a physical `Fn` key in Swift

I want to use the Fn modifier key if available, and if not, offer the user a different modifier. But only offer it to users without a Fn key. Some external keyboard do not have a Fn key.

Is there any way to detect whether the active keyboard has a physical Fn key?

(I am not asking how to detect a Fn key press)

I'm looking for a solution in Swift that works within a sandboxed app.

I already tried asking on Stack Overflow.