My dad is acting suspicious (Cheating on my mom)?

Well....things have been a little weird with my parents lately....and it just doesn't seem right anymore. My dad goes out late at like 12:00-5:00 for example..and sometimes I think hes being unfaithful. Like tonight..he just received a phone call on his cell phone, and put the phone down until I left and it seemed like he was whispering. You might think I'm a little rat for saying this, but I unlocked his room and it was a complete mess. Interestingly enough, I found a lot of stuff that would lead to my suspiciousness. CDs, tons of condoms, numbers? PAPERS....with a bunch of stuff on it. He also started to chat heavily on the internet (He is computer illiterate..but I guess he learned how to use it). What should I do? ...my mom barely knows anything thats going on cause shes always at work..and Im the only one observing what hes doing. Should I ask my mom if we should follow him one night? I just don't want my mom to get hurt..