Before sending a mass message to all your subscribers, we always recommend sending that message as a test message to yourself, and even your colleagues. You can then see how your message will look on different mobile devices, and you can make changes before sending to all your mobile subscribers.
When you are using merge tags in the message make sure you review how merge tags work in a test message:
Existing Subscriber w/ merge tag data - you will see the data
Existing Subscriber w/ no merge tag data - you will see what ever you decide the fall back will be for that message
Number for someone not in the list - they will see the merge tag (e.g. {FIRST_NAME})
For instructions on how to send a test message, see the video below.