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 ocean 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"

From a prior thread:
From Philosophy to Religion wrote:Sometimes you have a very metaphorical way of expression and I don't always understand it. You wrote, for example:.By using what I call "Resolution Debating" an entire forest of truly rational thought can be generated wherein every angel is named and everyone can find likemindedness all within structured Science
What do you mean by "angel" in this context, or how do you define it? I should have asked you already when I translated the "Three angels of truth", but then I thought I had an idea what you mean, but couldn't really explain it. I never thought about that term very much.
An "angel" refers to an "angled branch of thought" or more simply, an idea or scheme. Quite often a con man is said to have an "angle" .. stemming from the same root concept, an "angel overlooking his plans" - a scheme. A Catholic nun would wear her "habit" so as to protect her from the "schemes of men"/"angels" (the very same reason women do not walk around showing too much of their body or masking their face ... not counting the possible embarrassment issue).
Scripturally, a "tree" is what today you might call an "orderly index of ideas" that branches from one central idea (directly related to the famous "Tree of the Knowledge of Good and Evil"). And the original DOS PC operating system ("Disk Operating System") referred to its index of topics on the disk, not as "folders" but rather as the "index tree". An overarching central idea is also referred to as an "Archangel" (such as capitalism, socialism, Rational Metaphysics, and so on.
Now with all of that in mind, what I was trying to relay to those who could have possibly understood (very few), is that RD ("Rational Debating") automatically generates a tabled index of branching thoughts/ideas/angels such that every individual can see where their beliefs fit best and who their like-minded friends very probably are. Each initiating thought is referred to as the seed of a "tree" (scripturally and technically) - a "seed thought" or what some have referred to as a "pregnant idea".
When RD is the common practice among merely a few people (or even merely one with a good imagination and interest), many branching trees get formed from initial thoughts - an "entire forest" of such branching thoughts. Basically every hypothetical, every "what if", could initiate a new tree of thought.
These ideas and concerns go all the way back to the stories of Abraham (an ancient (somewhat outcast) philosopher) and the days of Genesis.
Angels were not originally mythological beings. Ignorant people's imagination (with the help of inconsiderate, if not evil, people's help) made them into mythology (just like everything else). Trees and angels have changed only in their name, not what they have always been.
There is nothing, absolutely nothing, that is a part of religion that wasn't born in that same way:
Someone's ontological philosophy into many people's misrepresented religion.
And:
RM Fundamentals wrote:An ontology is an understanding of existence and is built upon predefined concepts proposed to be useful in the long run. Many varied ontologies can be built and be useful but can only be valued as true if they conform to the following stipulations;A) Consistent within the ontology
The word "science" means "the knowing". Rational Metaphysics is a method involving Definitional Logic, Scientific Methodology, and Resolution Debating for constructing ontologies that are necessarily true and empirically and logically verified. One essential definition for any ontology is its definition for existence itself. Objects and principles are then defined such as to build a complete coherent understanding, an "ontology".
B) Comprehensive in including details
C) Relevant to the needs at hand