Go into your self serve account online and you can check off an option to have text warning sent to you when getting close to your data limit. You can also set up a warning and cut off in the settings of your phone if you have an Android device.
Only if you have it set. You can check on your self serve.
Note that the notification does have a delay sometimes so it's not perfect. Your best bet is to set a data limit in your data settings if you have android. For iOS, check out My Data Manager