This is how objective fact is made objective: comparison. When I say 'The Eiffel Tower in in Paris, France', this is an undeniable and objective fact. You can't refute it, unless you compare it to another statement: 'The Eiffel Tower is in Berlin, Germany'. It could be. The tower was probably deconstructed and shipped in parts and re-assembled there. The only way to ascertain this counter fact is to go there and prove it, comparing your facts against each other.