Without falsifiable evidence they speak.
There is no way of determining if it's truth or not.
I see that is the only way we can determine if it is Truth. Why, because we are not the embodiment of that Truth but the Messengers are, that is a consequence of being given such a claim.
This is what they face, it is recorded in the Holy books the struggles they face with the knowledge of the trust they have been given.
So if we are shown by a Messenger what it is to be the embodiment of Truthfulness, we will be able to judge that against our own actions.
If we are told Truthfulness is the foundation of all virtues, then we can judge that against our own actions and against the actions of humanity as a whole. The Messengers will be the standard of that virtue.
Anyway, in the end we need the virtues to unfold for humanity, to enable us to embrace a unity of purpose. So let's hope that truthfulness wins through.
Regards Tony