You can always look at your AT&T phone bill. It will have every text and phone message on it and from what number it came from. Then you can investigate any numbers you do not recognize.
Not to get into it too much on a Public Forum, but I would just talk to him about your concerns that you all are "drifting" apart. If that is how you feel. This from someone who has been married 20+ years and 3 kids. Communicating can many times help any situation. Just my 2 cents. Good Luck.