CIC does not contact or conduct interviews with friends/family, nor do they do surprise visits during the application phase (or even if they did, many outland applicants are living in different countries anyways).
There is practically NO WAY AT ALL to have definitive proof a relationship is a MOC or not. Unless the application package happened to include messages between sponsor and applicant stating exactly that, or if they openly admit it in the interview. Obviously this doesn't happen. So the only way to weed out potential fraud cases are to look for red flags or things that are outside of what's considered normal. Lots of it I'm sure comes down to the gut feeling of the visa officer.