If you would like to know how much data you have used, you can head to your carrier's website or download their app, log in to your account and see the information there. But you can save yourself the trouble and simply check the info directly on your device.
Here is what you can do to track the use of mobile data:
Go to Settings > tap Cellular or Mobile Data.
For iPad users: Settings > Cellular Data.
If you're using a Dual SIM iPhone, set one of your plans as the primary data number in order to see how much cellular data you've used.
By the way... You can turn cellular data off to make sure apps and services do not connect to the Internet via the cellular network. When cellular data is on and Wi-Fi isn't available, apps and services use your cellular connection. As a result, you might be charged for using certain services with cellular data.