How can I tell when the iPhone is actively using GPS versus only using region monitoring?
In Settings --> Location Services each application that uses location services is listed.  In that list any apps that are currently using locations services or have used location services in the last 24 hours includes a little arrow icon.

The icons differ based on whether they are currently or recently used location services, and if currently then whether it's a geofence or regular updates.

The icons and description for each icon are included in that settings panel.