The Three Angels of Truth

How does one know that he knows? 
- One of the most relevant questions in all Philosophy.

The default presumption is to presume based upon intuitive probabilities. That is how people knew that the Earth was flat floating in a bowl of water supported by an elephant riding on the back of a turtle... until Atlas came along.

An "angel" in scriptural lingo refers to an idea, thought, or strategy (similar to a con man's "angle"). And as it turns out there are three thoughts that provide proof of truth. There must be a unanimous vote of the three angels. The word "true" merely means "accurately aligned", usually referring to the alignment between a statement or claim and objective, physical reality.

Given a specific ontology (and avoiding the common presumption that everyone is using the same ontology), one can certainly know that he knows with certainty if he can confirm that his thoughts have the following attributes;

A) Consistency/Coherence
B) Comprehensiveness
C) Relevancy

But then, how do I know that with certainty?
Because nothing else is Relevant concerning a proposed truth (aka "I don't care about anything proposed as truth if it doesn't meet that standard") and I maintain that concern consistently and comprehensively. It is my Definition of "a truth" (thus "true by definition"). 

Years ago, I was surprised to see those angels appear in a small booklet given to a set of churches regarding the proper method for interpreting the Bible. The author expressed them as a means to check one's presumption of interpretation. I recognized them a little differently as not merely a means to know of a proper interpretation, but a means to know of the truth within any given ontology (the Bible being merely one and a different one than Science and thus one cannot intermix the elements).

How can you know if what Physics says to be true, really is true? Faith in what you are told by a media service (a mediator)?

Look merely for the definitions of the elements they propose and verify a unanimous vote of the "Three Angels of Truth". You will find that they don't know those angels very well and espouse some truths that aren't. And you can know it with certainty even without being a physicist ("don't mess with a good metaphyscist" ).