You'll have to ask your friend's friend if he has used the gun in a shooting. The main way the police can trace a shooting back to the gun is through ballistics testing.
Really, you are better off buying a gun through a gun store. If that gun happens to be linked to a homicide or attempted homicide and they do a ballistics test on it and it matches, the cops will be balls deep in yo bidness. Even if you are not found guilty, that time spent interrogating you won't be a picnic. And if you want to come clean, you'll need to dime out your friend and friend's friend.