1) Infidelity is not a reasonable grounds for divorce.
why do you need proof then? You say that you know she's doing it, so do you need the proof for yourself to convince yourself? Or maybe just to know the truth? Or to be able to confront her with it?
2) UNLESS you are a member of law enforcement working under a court order signed by a judge, or a licensed private detective then phone spying is ILLEGAL. And is against the usage agreement of the hofo forums. Thus, cannot be discussed here.
IF your name is on the phone account, you can try contacting the carrier and request the phone records. Some carriers can print out recent text messages, others don't bother with it.
Man, I'm sorry about your wife. But if you know for a fact that she's cheating, either confront her about it, do whatever needs to be done so that she doesn't feel like she needs somebody else, put up with it, or beat her to the punch and get the divorce lawyer first. Thats the only advice that we can give you here.
If only everything in life included an UNDO function....