You could use a smart group looking to see if their IP uses your IP scheme. As far as macOS goes I have an EA that reads their current network name and if it doesn't match our student network it puts them in a smartgroup and emails me about it. I also have another EA that reads their Preferred network list so I can see if they've been connected to a hotspot and then I can use a smartgroup to look for a specific hotspot ssid if they're causing a lot of problems.
