To check the balance on a T-Mobile phone:
You just need to go to the dial pad of your T-Mobile, dial #225#, and press the call button.
You can also dial #BAL# and press the call button.
After a few minutes, you will receive an alert message stating the up-to-date balance of your T-Mobile device
When you have only 20% of the data balance remaining or When you used 80% and 100 % of your data on T-Mobile you do get free messages from the service provider at each threshold which helps you keep track and not pay a huge amount on the extra data usage.
These messages are helpful as the speed declines or in some cases, the service completely stops in case of complete data usage.