Odds are very good that she is having an affair. Whether it's EA or PA doesn't really matter much. How would knowing the facts help you at this point? For many people, knowing for sure just undermines their efforts to DB. Plus, if you actually are wrong, accdusing her of it will be the death knell for your R.
That being said, sometimes having it out in the open will be a necessary step. However, there's real risk that your W will resent you for it or get mad at you for "snooping". If you feel it is necessary for it to be out in the open, try to find out "by accident" or by someone calling you.
Do you live in a state where adultery matters in a divorce? (It doesn't in most but may in some). If that's the case, you may want to quietly gather documentation in case you need it in the future.
Otherwise - I would let it go. Focus on being your best you and on DBing. I'm grateful I didn't find out about my H's affair until after I'd been DBing for a couple of months.
