I want to see if there is mobile data activity. How can I do this using the WinRT api (c#)? I can check if the device has active mobile data, ethernet or wifi connections, but can't seem to check if any of those connections are currently sending/receiving data.
You could use http://msdn.microsoft.com/en-us/library/windows/apps/windows.networking.connectivity.datausage.aspx and check the bytesReceived property. If it changed, data was just transfered. See the linked Quickstart for more information on how to use this.
Other than that, I don't think there's a specialized API for that.