How do I catch "my boyfriend" sending and responding to online personals?

My soon to be ex boyfriend asked me to look something up yesterday on craigslist. When the site loaded I noticed that the men seeking woman section had been visited, as well as casual encounters. I confronted him and he said he was lonely when I was at my apartment or at work. The choice to live seperately was his. During the first few times I met him he was still talking to his ex on the phone alll the time, just as "friends" though. Later an overly confident fat woman a couple apartments down would give me dirty looks or snicker everytime she'd see me. Another time I was going into the apartment building with my dad and 2 fat mexican prostitutes were coming out when we were going in. They acted similarly to the other woman laughing like they had some secret inside joke.