It's fairly easy to find out how much data has been used.
Please do the following:
Open Settings > Cellular or Settings > Mobile Data.
*iPad users might see Settings > Cellular Data instead.
Scroll down to see which apps are using cellular data. If you don't want an app to use cellular data, you can turn it off here. When mobile data is off, apps will only use Wi-Fi connection.
Moreover, you can still send and receive SMS and iMessages if you're connected to a Wi-Fi network.