How do I unlock my hubby's screen on his computer?

Ok well I recently found out the ho has been using an escort service and spending all our money! I'm really mad because I saw the picture of some skimpy chick in a corset with her hanging out, and it had a phone number so I called, then the screen locked before I could save the number. How do I get into his computer? I've already called the police. Its a macintosh, and his hint is his picture and "motorcycle dude"