Look at how these documentaries represent their "truth"... do they use scientific/technical jargon with qualified scientists/experts to ENDORSE their truth?
Examine the way in which they deliver content to you as a responder in such a way that you accept their truth - perhaps shots of letters, evidence etc. or computerised/animated graphs and simulations to again, ENDORSE their truth.
You also have to think about the way your texts explore, refute or endorse the representation of truth.