Using Face Detection

I´m very happy with the App, but it´d be great if someone could point me in the right direction about face detection.
I enable it in settings but nothing happens. How do I know if it´s working?
Is there any addicional requirement? The phone I´m using is quite old, but works great for a fixed panel in a room, maybe is an Android thing?

There is a camera test button in the settings which will show you if the face is detected.