It's unfortunate that people stray in relationships, but you should know it's not entirely uncommon.  Right now there are many women just like you agonizing over their suspicions and how they should see them through.  That said, I strongly advise against sneaking into his personal devices to get the answer. The most important thing in a marriage is communication.  If those walls start breaking down, then everything else will begin to crumble as well.  Sometimes, certain circumstances cause a strain on the relationship and it can be easy to fall into an affair.  It takes courage to steer clear of getting involved, either emotionally or physically, with someone outside of your marriage, especially if it's been 10, 15 years.   

Tiger Woods' wife reportedly went ballistic on her husband after rumours of an affair surfaced.  While we can't blame her for her freak-out, it doesn't mean she dealt with the news the best way. In fact, based on Wood's voicemail to another woman he reportedly became involved with, it seemed his wife Elin was also driven to prying.

Sneaking around and stealing your husbands phone or gaining access to other private devices will only create a greater wedge between the two of you.  Eventually you're going to confront him about what you may have found and his trust in you will fade fast, despite that fact that he has forsaken your vows. You're probably thinking that his actions only led you to such extreme measures, but believe me when I say that by snooping around, you're only equipping him with ammunition in the event a disagreement arises from this mess.  At no point should this issue become clouded with your unfavourable scheming to get to the bottom of his unusual actions.  Confronting the issue head on is always your best bet. Sit him down and reason with him about why being truthful is important. Explain you're certain he's up to something and the only way to save the marriage is to come clean now.  But, you must be prepared to hear painful details or worse. Be prepared for him to admit to an affair and a revelation that he's unhappy.  In some cases he may reveal his wishes to terminate the marriage.  Sometimes when secrets like this surface, true feelings are also uncovered.  Sometimes, moving forward just doesn't seem like a viable option.

Now, before you take a golf club to his SUV, you need to consider the following. Not all marriages end in happily ever after. Marriages are tough and take work. As the years progress your love should strengthen. But just as your bond becomes stronger, intimacy may weaken, leading some to stray. It doesn't justify his potential actions but it's a reality nonetheless.

You cannot consume yourself with what ifs and various other scenarios you believe are taking place. That will undoubtedly drive you to insanity. Just look at Tiger Woods' poor wife who reportedly became so consumed with her husband's infidelity that she ran after him so much that he had to hop in his car and make a getaway, albeit unsuccessful.  Do not allow these suspicions to fester.  You know your husband; if you lay it all out for him and he denies it and you still think he's lying than you need to be true to yourself and take whatever measures you see fit. Perhaps counseling would help. Clearly if you believe he has been unfaithful there is enough of a divide in the relationship to merit that you both seek help. Some women are able to stand by their man when something like this happens and you need to figure that out if you're prepared to do so if he admits to an extra-marital affair.  Determine just how much are you willing to put up with and if the years you've put into the relationship are worth the agony it will cause to stay with him. Whatever you choose, it must be calculated, and discussing your options with your husband is a good idea.  Good luck on this difficult journey.