How can I find out if my wife is using social media to cheat on me?

A little over a week ago, I found out through phone records that my wife was in communication with her ex boyfriend. I realized afterwards that she stopped using basic calls and texts to communicate but I do notice she guards her phone particularly her Instagram account. I'm thinking she is using the message features of possibly Instagram or even WhatsApp/Viber to talk to her ex. Is there a way to find out?