Through DHCP, the DHCP server can set different DHCP-options to notify clients what type of connection it is. For example: Android Phones set up to be a hotspot will set DHCP option 43 to a value indicating that it's a metered cellular connection.
I had no idea DHCP could even do that. What do Android devices do on unmetered cellular connections? How do they know if the connection is metered or not?
In android settings you can set if your connection is metered or not, and set thresholds / warnings.