Confront the man. Decide what you will do about adultry if proven. Ask God that the truth be revealed. Hire a dective if affordable, or talk to friends about what they might know. Are you willilng to forgive her if she has done this? Then tell her so if yes and then go for councelling. If not then tell her that you can no longer live in doubt and then you both go for councelling. Watch the movie fireproof, read the book and work on saving this marriage. Do not let this eat you up. You need outside help and must seek it out. Your marriage is worth your best effort to save it.