I think it would be a really bad idea to track your wife's text messages to be honest, as she is sure to find out and will be very upset!
Of course, the other side of this is that you can't legally check other people's messages anyway so if she found out, and things got nasty, you could get into a lot of trouble.
Do you have a reason to check your wife's messages? Perhaps you think that she might be cheating, or saying inappropriate things to someone... there are other ways around this.
Any worthwhile relationship is built on trust, if you don't have that then everything starts to slide. If this is just a jealousy thing or a nagging suspicion, try to put it out of your head. The truth is that a lot of us get these feelings at times but to act on them the way you suggest, would be a mistake.
My suggestion would be to take your wife out for a meal, show her how special she is. Maybe she is acting differently because she feels you have changed and is worried that your attention is elsewhere.
When the moment is right, sit down with her and talk things through. Tell her that you have suspicions that are nagging you and although you trust her 100%, you need to know that she is happy and feels loved.
This should lead to a good talk about anything that is on her mind - and yours. You will feel so much more relaxed after clearing the air, and you won't have had to do anything sneaky to get to the truth.
Good luck, remember flowers and chocolates work well too ;-)
