Android
Step 1: Open the 'Settings'
Step 2: Tap 'Connections'
Step 3: Tap 'Data Usage'
Step 4: Tap 'Mobile Data Usage'
Step 5: The top of the screen displays your data usage for the current month (as determined by your billing cycle). Tap the 'Date' to see previous periods
Step 6: Scroll down and tap any app to find out how much data it uses and to control whether it can use Mobile Data, Wi-Fi, or both
Step 7: Go back to 'Data Usage' and tap 'Billing Cycle" to set the date when your plan resets the monthly billing cycle.
iOS
To view overall usage, go to 'Settings' > 'Cellular' or 'Settings' > 'Mobile Data'. If you're using an iPad, you might see 'Settings' > 'Cellular Data' instead.
To view the Internet quota used by each app on your phone, scroll down. If you don't want any app to use cellular data, you can turn it off for that app. When cellular data is turned off for that app, it will only use WiFi.