I have always been a proponent of philosophy that if you think your girlfriend is cheating on you then chances are she is. The philosophy is extended when you actually have hard evidence (ie not just circumstantial stuff). Once this happens then I say it is almost 100% certain that she is cheating.
Thus, after learning the following facts I believe that my girlfriend is cheating on me.
The Background: Over the years the relationship with my girlfriend has deteriorated. Perhaps the biggest thing which has gone is the passion we seem to have for each other. While I am not the most affectionate of guys and my girlfriend is certainly not either. Without going into details lets just say our sexual encounters with each other have diminished substantially.
The Evidence: She went out with a few of friends in NYC and called me at 4:30 a.m. saying she was on her way home. Clubs and bars aren't allowed to serve alcohol after 4am, but usually any bar wouldn't be open after 3am. Usually only a guido club would actually stay open till 4.
Then the next day we are lying in bed and her phone starts ringing and it is on my side of the bed. she starts flipping out trying to grab the phone before I look at it. So naturally I have to look. It turns out to be a text message from her ex boyfriend asking if she is alright. Now lets preface this by saying she never told me her ex boyfriend was going to be at the bar she was going to.
Add to this the obvious fact that at one time she was attracted to her ex (otherwise she wouldn't have been with him) makes it even more likely that something happened.