How do I track the usage of my Roaming Data plan?

You can see the how much data you have used, as well as per-app breakdown directly in from your phone settings! 

Please refer to the instruction guide for your specific device but here is a short set of steps for both iOS and Android

For Apple

1. Go to Settings on your device.
2. Tap Cellular or Mobile Data.

There at the bottom of the screen you will see Data for Current Period and Current Period Roaming

For Android

1. Open the Settings app
2. Tap on Connections.
3. Tap Data Usage.