Truth is a much-debated concept.
There is (presumably) an objective underlying reality which is the same for everyone. But perspectives on it, and perceptions of it, differ.
Psychologists talk about qualia: sense perceptions which we know have an objective referent, e.g. what red looks like, what celery tastes like. But we can never be sure if another person's experience of qualia is exactly the same as ours.
The universe is infinite and we are finite (located in a particular place and time). So our perception is local and finite. That includes our idea of God. Hence the story of the blind men and the elephant.
So, we don't automatically know what the truth is about a lot of stuff, apart from obvious physical facts, such as seeing one person kissing another. We see them kiss, but we don't know their motivation for doing so. Perhaps even they are not fully aware of their own motives, though they are aware of most of them, and have a reasonable idea of each other's motives, and whether the kiss signifies a romantic relationship, or something else.
When looking at a theory, physicists and mathematicians use its beauty and elegance as a test of its truth.
Science, of course, uses the scientific method to find out whether something is true or not. The scientist carries out an experiment which either confirms or denies the hypothesis. This works well for statements which are falsifiable (can be confirmed or denied through experiment), but not for ones which aren't.
So given that we can't know everything, how do we ascertain what is true?
Even when you accept the doctrines of a religion, there must have been something about them that made you decide at least some were true and you would trust the rest. (It is worth reading Godless Morality by Richard Holloway on this issue.)
So, one can look at the intentions behind a policy or practice or belief, or its effects, to decide if it is valid.
One can look at the internal consistency of a set of beliefs: do they contradict each other? Do they follow logically from their starting premise?
We can also look at their external consistency: do they contradict known facts about the world? Are they consistent with other religions and philosophies? Example: a religion must have a theology to account for other religions (and in my view, preferably not that all other religions are inspired by the devil).
Different types of truth:
- objective fact, something actually seen;
- something taken on trust, because we know the methods by which it was discovered, e.g. molecules;
- metaphor and analogy: a good description or model;
- mythopoeic truth - something that rings true; a mythological story that conveys something about human nature or the way the world is. It is not literally true, but it rings true.
- finite and infinite perspectives.
I find the Greek for truth interesting and poetically apt: aletheia. The opposite of oblivion.
Wittgenstein said "Whereof one cannot speak, thereof one must be silent." (Tractatus Logico-Philosophicus). Even if one cannot remain silent, one should be aware that conclusions reached are provisional and transitory. That's why the Dalai Lama said that if science proved that reincarnation does not exist, he would stop believing in it.