If this is a serious question, then contact FBI or Local law enforcement. But before you do, I would make DAMN SURE first. If this is just a matter of you not liking how your neighbor "smells" or looks, I may suggest getting a life first, unless you have plenty of evidence to back up your "suspicions". (Just because a neighbor doesn't mow his or her lawn regularly or join in the local Bridge games or Tupperware parties doesn't make them even POTENTIAL criminals.) Of course, you may want to add more details to your question if you would like more or better answers.
It is not the obligation of unskilled civilians to discover proof. Leave that to the professionals. If you suspect a felony is being committed, you are required to notify the authorities. You can do this anonymously; and the sooner, the better.