How to confirm WiFi data is being used, not data plan?
I use a Galaxy S5 running Android 6 (Marshmallow) and my phone has different icons for wifi and data.





My phone also has little arrows that turn white if there are packets are being uploaded or download.

You can see data usage by app in the data usage setting.



And if you find that an application decides to use a lot of background without telling/asking you, you can click on it, and disable background data.