Premise: Not breaking any new grounds here, but laying it down as a reference.
Something is true if it’s real. Everything that we conceive is the fruit of perception, which is an intake of signals from our sensory abilities, such as vision and hearing. For all intents and purposes, there is no objective reality. Our mapping of reality is limited by our sensory abilities, which are limited by nature. Reality is also how we integrate those signals into our model of the world.
To try and establish a common base reality, one should follow two major rules:
Reality is defined by consistence. This at the root of the scientific method. Nothing can stand on its own, unless it’s consistent with the rest of the established theories built on observations. This does not mean that established theories cannot be changed, but they should only be changed or amended as long as the new model adds new details that gives a better understanding of nature.
The observer should question the environment if there’s a sense of impaired mental capacity of the self. This is to avoid dream-like states of mind. When dreaming, belief is usually momentarily shaken by the fact that one is unable to perform trivial mental exercises, such as actually looking at a screen with code and being able to edit and debug it. This is a sign of the fact that the brain is busy trying to generate its own reality instead of simply processing inputs from the real world. In popular culture this is sometimes defined as pinching oneself to see if there’s a sense of pain. The idea is to perform a sort of brain pinch to see if there’s struggle to achieve a level of mental acuity that is known to be possible.
Of course, these rules are relatively vague in themselves, so for practical purposes they are guidelines, but I believe that they are what one should strive for to establish a reality to work with.
It’s very easy to claim to be consistent. In fact, one should always apply some self-doubt at certain junctures to rethink on whether or not he/she is indeed being consistent and see if perhaps there’s a deeper level at which this may not be the case anymore, such as when added details would negate the consistency of thought.