I wouldn't attempt to figure out if there is another woman unless knowing for sure will compel you to make a decision one way or the other.

Affairs are frequently discovered at some point. He'll text the wrong phone number, or a friend or acquaintance will see him with another woman and tell you. He may even intentionally "slip" and let you discover the affair, because you will either leave him or your reaction will "force" him to leave you. (That way, he gets to try to alleviate any guilt he may be feeling over his actions.)

Me: 44, Wife: 39
M: 17 years T: 20 years
Bomb on 08/25/09
1/13/10: MC started
1/28/10, 2/8/10: More bombs
8/28/10: Wife moved out
No talk of D, no movement

"Every day is another chance to get it right."