You can check your remaining data anytime in the Thrive Mobile app or the web app.
In the Thrive Mobile app (Android)
Open the app and go to Account → Data Usage. You'll see how much high-speed data you've used and how much is left.
In the web app (iPhone or older Android)
Go to webapp.thrivemobile.com, sign in with the email you used to enroll, and open Data Usage.
Running low?
If you're close to your limit, you can add more high-speed data from the same screen. See Buy more data (top-ups).