Well if she is, clues will be right in front of you. If your asking this question you probably already know the answer. I would start by varying your schedule. Pop home, or call at a different time, occasionally if necessary listen in on conversations, or has she taken a overly keen interest in her appearance when going out alone? If questions are still uncertain in your mind, there are remote tracking devices you can install in her car to follow from your home computer, similar to a gps. Good Luck. I hope you are wrong and just paranoid.
If you legitimately suspect your wife is cheating, hire a private investigator. Don't do the cyber stalking stuff like installing a GPS device to track her. If she finds it, you might be in big trouble yourself. In Washington state, that is a felony. An investigator will probable get you solid proof for 500-1,000 dollars.
if shes started acting different, changes in the way she treats you. not interested in things between yall that she was before. ask her straight up, i think honesty is the best policy. if shes not hiding anything then theres no need for her to refuse to look through her phoneright then and there. no "spy stuff", that could get you in a world of trouble. private investigator, or you just become an investigator yourself. and do as said before...pop in differently, get off an hour early from work, go home for lunch. take a day off, leave like you are going to work, give it an hour or two then just return home. sick day or somethin. ive been cheated on SO many times, itsna painfull situation. ive always been the loyal one getting crushed. I hope all works out for you and its just your immagination. good luck.