Is there any way to detect iOS 9 low power mode programmatically?

I know the user can detect low power mode by observing the colour of the battery icon, but is there an API call that allows a program to detect low power mode?

Yes, you can either add an observer or query it directly.

https://developer.apple.com/library/archive/documentation/Performance/Conceptual/EnergyGuide-iOS/LowPowerMode.html