Can I weigh in on that?
It's very rare to encounter those kinds of cases for the first time at the border and for a border services officer to be able to distinguish the indicators that would lead to an ultimately founded suspicion that he's dealing with a fraudulent marriage, because the filtering is supposed to have been done in advance. By the time a couple shows up at the border with a permanent resident visa, all the checking has been done.
We are aware that certain programs have been particularly targeted for fraud. We at the CBSA, through our intelligence component, develop guidelines, papers, and indicators that are used by CIC visa officers abroad to do an initial screening.