Not to put TOO fine a point on the matter, but; if you feel that you need a DNA test, I'd say you've already got trust issues. Deeeeep ones. And those never develop in a vacuum. Truthfully, I have to say that trust sounds like only one of many undiscussed, and therefore unresolved, issues.
Maybe it's time you and your old lady really started to communicate. It may get ugly, but ugly truth is far better than painful suspicion.