I have no better idea than to encourage thinking, using the intellectual power God has given to any of us!
This may vary for different people how they can find the truth, but for the people joining the StackExchange who are commonly educated with a good amount of knowledge I will suggest a gradual (step by step) but rigorous proof! Uneducated people may even never need such proofs, they have not learned and practiced to repeatedly ask why? Why? and Why? They easily trust in scholars, in parents, in family and friends, in news from radio and TV and internet, less suspect, less question, and more move with their hearts and hopefully good will. But for people here (and similar to them off this site), it is more than recommended to provide ourselves with reasons and rigorous proofs, otherwise today we are satisfied with an answer and tomorrow will be another challenge with a new question! We can classify the questions like this:
Those that are fundamental about religion: These cannot be answered from Quran or Hadeeth but merely reasons. For personal reasoning one can also trust in his/her inside feelings and understanding, although reason is reliable anyway in challenging with doubts.
Those that are fundamental to our sect but not religion: These can be addressed based on reasons and Quran (in Islam of course, and similarly other books for other religions) only, but still not hadeeth or scholarly opinions depending on assumptions from a specific sect.
Those that are neither fundamental to our religion nor to our sect: These can be addressed like most of the questions are answered now in Islam.SE, based on Quran, Hadeeth, reasoning, scholarly opinions and etc.
Among these three classes of questions the first two is of interest to the present question. The reasoning part of the answers is common in both of these classes. So let me clarify my idea about this issue:
When a religion/sect can be considered reasonably righteous?
If a religion/sect is to be the righteous one, the one that we are expected to admit currently, then it must be the true one, that is others should be the fake ones. There are two ways to determine a right answer among the available answers: the direct method and the indirect method. The direct method tries to prove why a religion/sect is righteous, usually difficult. The indirect method tries to get rid of the other wrong answers, the remaining one would be the right answer. That which method is best depends on the difficulty of what we are to prove directly and the number of available answers that should be proved to be wrong.
In Arabic, truth (حق) has the same root as Mutihaqqiq (متحقق) which implies "existed or occurred". Baatil (باطل) is somewhat its antonym, something that cannot exist or occur. Like in axiomatic mathematics that a system of mathematics is existable only if it is self-consistent (otherwise in a single system we will have e.g. two contradicting theories, so that based on a same assumption we will have two contradicting results, a statement and its contradiction will be equally considered true in one system, self-contradiction, quite illogical) anything that is to exist in a rational (logically existable) universe should undergo its logical rules, otherwise the universe would be self-contradicting and not existable any longer.
Let me bring you an example. If we know that an idol is not a god, we can equivalently call an idol a fake god (الها باطلا). According to Islam, e.g., God is the only one existing god, so He is Haqq (حق). And He is the only Haqq, Al-Haqq, so that nothing exists (independently) along with Him, and if we now exist our existences are dependent on His existence, we exist only to the extent He has given us existence (creation). Also equivalently, any existence in this universe is truth (حق) and should be rational (as it has been already occurred really in the rational universe, it is متحقق, if it was not rational it couldn't exist alongside the other existences there). An idol is truth as a piece of stone, wood or whatsoever, but is fake as a god. A stone being god is not rational (referring to the definition of a god, and a stone or sculpture or etc.) as it cannot exist as what it is not created to be. A man is truth as a man not a woman and vise versa. A cat is truth as a cat and not a dog, and infinitely many other examples.
Anything that exists is truth and truth is rational (based on the underlying logic of the universe which is container of such existences) and will never cause in any contradiction with any other existence in the same universe. To put it other way in this Existing universe only rational objects receive any credit for existence and they will exist only in their rational form, and any rationality needs self-consistency, and truth --in contrary to lie-- is always self-consistent. All these introduction will conclude one statement and that's it: "A true religion/sect cannot contain any paradox in it and cannot cause to any contradiction with any other existence in the same universe. Every aspect of truth should always go well with other aspects of the truth!" So let study our religion/sect. Is there any paradox in it that is not resolvable at all? [
we should always bear in mind that our knowledge of truth is epistemical and probably lam in most of the occasions, so something that may be recognized as a paradox may be resolved sometimes later, but there are paradoxes that can be proved to be un-resolvable and such paradoxes can be the basis of the present discussion only!] If so, then there is a problem, we should try to find the source of the paradox and then revise our assumptions (some of our beliefs) for the whole system of our beliefs to seem rational again. If there was no way to revise our beliefs in the context of the religion/sect that we presently have then we should be brave enough to convert!
Let me bring you some examples of paradoxes:
God, the Omnipotent according to the definition, can do anything with no restriction! This is wrong, as He cannot create such a heavy stone that Himself cannot lift! So there should be a restriction on His creation. That he can create only the existable (rational according to the logical rules Himself has set for the universe we are talking about) things. He is then not omnipotent? Yes He is. But creation has two sides, one side the Creator and the other side the thing to be created. Each side that has a weakness the weakness would appear in the creation anyway. Allah has no weakness (according to the definition) but the thing to be created has weakness in that it is not existable, so the result would be God not creating it!
God, the omnipotent, can in principle be incarnated in a flesh, like Christians think about Jesus --peace be upon him-- or some other people may think about it in the Day, after this life. No, again similar to above, according to definition God cannot be limited to anything. His being limited will be an imperfection and contradiction to Him being a god.
Jesus --peace be upon him-- was a god (like the son of God or God Himself) but at the same time history tells us that he was worshiping father God, doing prayers and fast a lot. Again is a paradox, since a god according to definition is independent and cannot be affected by any other existence, so does not pray or fast.
Islam.SE can be even more informative and useful for others if people can find, question and answer around such paradoxes. This will help all of us to reinforce our beliefs!