You can mark devices as used for development in XCode organizer.
What does it actually do internally on iOS device? Does it set some kind of flag? Does it install something?
Is there API on device (including private) to check whether it’s development or not?
Is there API on device (including private) to mark device as development?
I think I found small piece of information here: What does the "Use for Development" button in Xcode Organizer do to an iOS device?
However it covers only partly a question what does actually happen with iOS device.
Use for development will copy the UDID, upload it to your itunes connect account, update profiles and allow you to refresh development profiles to be used with your device. These profiles will be installed on the device when you do builds or ad hoc installs.
Its a huge convenience compared to doing all steps manually.