> You can setup a campaign where you pay for comments
You cannot setup a campaign where you pay for comments (https://www.facebook.com/business/help/1438417719786914#). But maybe you mean other user generated content like messages. You ought to be able to figure out pretty quickly if those are authentic.
My bad, I guess I was thinking of engagement campaigns and yes messaging campaigns
Do you care if they are authentic? Probably not. You are in the business of getting clicks on an ad. Your client has to worry about actually converting that to product sales. Thats their problem and not yours as the ad firm.