Remote keyboard with USB dongle under Linux: how to detect keyboard is ON?

So, problem is simple: I use a keyboard in an linux app (developed in Go) and am trying to figure which keyboards are connected, through Bluetooth or USB. Using a combination of listing event files under /dev/input and using udevadm info and evtest on them (root suid), I could figure that perfectly fine. The main issue I still have is, when my remote keyboard (seen as USB, since it uses a USB dongle) is off, the keyboard (actually the dongle) is still visible and the corresponding event is there, but obviously I can't know if the keyboard is on or not. Since I'd like to remind the user to swit

Question by Phil H on Super User, licensed CC BY-SA. We show a short excerpt and link to the original thread as credit.