Conduct random physical device inspections... have her give you the phone, unlocked... and look at them on the phone... or... link her phone number with an instance of Message on an iDevice you control...
iOS and OS X: Link your phone number and Apple ID for use with FaceTime and iMessage - Apple Support
How can I link my iPhone number with Messages on my iPad, iPod touch, or Messages for Mac?
When you sign in to Messages with your Apple ID on your iPhone, you link your phone number to your Apple ID. If you then sign in to Messages with the same Apple ID on your iPad, iPod touch, or Mac, Messages will link that device to your phone number as well.