You can either call or text 611 and ask or sign into your MyTmobile account and check there.

The steps to check your balance on the app (in case you needed them)

1. open the app and log in
2. On the manage tab, scroll down to just about the bottom and select Device payment plans.
3. You should have a list of the devices currently being financed. For more information on the specific line, tap the carrot next to the line to show the details. 

Hope this helps!