How to be confident mobile data is off
The standard Android Settings->"Data Usage" allows you to set warnings and limits for mobile data usage. The screenshot is from Android 6, but I recall having something similar in Android 4.x. 

This won't give you a visual indication of data usage on your home screen, but it will give you a warning (I don't remember what this looks like) and it will cut off your mobile data if you hit the limit. 

The advanced settings also allow you to toggle whether data is enabled on roaming connections.