Quick response: Of course I believe there are some paid "shills" out there. But wether they are on GLP or not, or wether it does a difference ( f(A)=f(C) ) is another question.

EDIT: For distinction of "paid shills" I would say evasion of questions, ad hominems, twisting facts, cherrypicking, spam or other possible manipulative/evasive behaviours may be an indicator of weak argumentation or intended manipulation.
But again it does not make a difference if both participants have the same means of getting their word (arguments) out to an independent audience.
Have a good one !