How can you prove adultery?? Is a physical witness enough??

[justus3/me]

In the Commonwealth of Va, Exactly what proof do you need to prove adultery?? Is a physical witness enough? I have a court form, where he put his girlfriends home phone number down as his own home phone number. His girlfriend told me that all of his money is going into her account. No doubt that was in an attempt to keep from paying child support, only it won't work. But what proof do you need? Does a private investigator normally pan out results in situations like this?? Or would it be just a waste of money??