I'm not sure why you need hard evidence unless you plan on taking her to court. I think reading something with your own eyes is enough evidence. If she slipped once she probably has said the same shit to others and possibly met them in person, cut her loose as without trust she is not your girlfriend just a fling, next time create 5 new accounts and fax the results to judge judy for an emotional distress claim.
generally when you go spying you already know what you will find or else you wouldn't be looking.