Well we know Google can classify what spam sites are.
And assuming they could (despite your arguments against) determine the method of monetization on a site, then simply compare the models they see on spam sites versus another metric which would track the user's reaction to certain types of monetization models.
And assuming they could (despite your arguments against) determine the method of monetization on a site, then simply compare the models they see on spam sites versus another metric which would track the user's reaction to certain types of monetization models.