You know how it is these days - it's pretty much impossible to command anyone's full attention. Think about it. When was the last time you had a conversation with someone without them checking their phone or tending to personal errands? It has become pretty socially acceptable to send a text here or there, or even check Instagram or Facebook while you're hanging out with someone in person.
If you have your lady's undivided attention whenever you're together, it's a sure sign that she's madly in love with you. She won't even be thinking about her phone when the two of you are together. She is way too focused on the amazing conversations you always have. Likewise, the thought of distracting yourself with Facebook or something else on your phone doesn't even cross your mind when you're with her.