We can't. But we can, by learning about Judaism beyond its superficial characteristics, see glimpses of its many lowercase-truths: its reflexive truth, its genetic truth, its referential truth, its subjective truth. And these put together come as close to "Truth" itself as humans are liable to arrive in this lifetime.

Is beauty true? Keats's doomed Grecians certainly thought so; perhaps, then, it is no coincidence that a kosher Torah scroll may be written in ancient Greek. For the Torah is beautiful; its many, many parts fit together perfectly, even where they seem not to do so at first or at second or even fiftieth glance. Is this evidence of a Divine origin? Maybe. Is it evidence that the Torah is internally true--that is, that it is completely integral, harmonious, coherent, and pure, like the notes of a correctly-tempered scale, or Da Vinci's ratios? Yes--and arguably it is better and truer than any of these, for its beauty is composed of far more dimensions of depth and historicity, on a far grander scale, and with a sweep which reaches farther, from the first flickers of light until the last breaths of humanity. As such, it had the gravity to inspire both Bach's partitas and Da Vinci's paintings.

The genetic and referential truths of Torah are, in my opinion, the simplest elements of its Truth. These have to do, respectively, with Torah's historicity and its power to correctly predict and reflect the world. 

Torah's power to correctly predict and reflect the world has been discussed interestingly in other answers. There are numerous, very striking examples of Torah's uncanny and precocious accuracy in describing phenomena of nature, biology, chemistry, astronomy, and even the physics of the last century--to say nothing of its brilliant and subtle philosophical assertions, whose truth becomes powerfully clear through lived experience. All these "discoveries" carry with them a certain magic, magic having both edges of spiritual inspiration and susceptibility to bias: the more one learns, the more truth and internal and external resonances one finds in the Torah. One runs into some truths that are merely felicitous and delightful; some that are astounding; and some that are profoundly moving. Arguably, though, these work upon the heart and soul so engagingly that the mind has trouble keeping pace, forgetting, at times, to present the ticker of things encountered that are *less* true, or that are patently false, or that are only true because of tricks and sophistications. 

But when one chooses to live by the whole Torah, the total of these truths points quite compellingly in a certain direction. Here, the combined evidence is greater than the sum of its parts; perhaps exactly this is what constitutes the individual experience of Truth. That direction is the good and proud and kind life that comes from Torah. It points to individual survival--to happiness, spiritual peace, health, community, stability, relationships, and pride--and, 'בעזרת ה, to survival through children, survival across generations, and survival as a people through havoc and adversity, moreover with great success, with money, with flourishing statehood, and with prodigious achievements and contributions in both humanity and intellect, B"H. This is the most basic evidence of one facet of Torah's truth, its referential truth as it applies to life--Torah describes correctly, and it prescribes correctly, and by its prescription, our people have lived on and on and thrived against unfathomable odds. 

Moving on to the Torah's authentic origins, its historical truth: The written and archaeological records supply, indeed, ample evidence of a naturally-enabled escape by the Jewish slaves from Egypt, followed by some unusual geological or meteorological event at Sinai. The idea that the basic events of this account were made up, that 600,000 people were initially convinced of the lies, and that the lies survived in their original form for 65 generations thereafter (because the fervency of this belief caused parents to impress it on their children at every cost) is difficult to conceive. Still, it is not impossible, given that other religions have achieved nearly the same with their incompatible accounts. But even this we must question: 1) Are their accounts really incompatible with ours? --Up to a certain point, no. And after this "certain point," the Jewish chronicle of faith essentially ends, whereas others diverge and go further. 2) Did other religions really achieve the same? No, because their tenure is much shorter. Only Judaism and perhaps some early forms of Hinduism (on which my lack of knowledge prevents me from commenting) have existed for anything like the length of human history. So the tenure of Jewish belief really is quite remarkable, and lends some credence to its authenticity. Humans have simply never believed anything for as long, and with such consistency.

How we jump from "is" to "ought"--from historical facts to the Law, and its current force on us--is, as always, harder. I admit a gap where it comes to the question of whether G-d spoke to Moshe Rabbeinu and said such words, or whether the Jewish people, astonished at the miracle of their liberation, and at the formidable evidence of G-d they saw at the mountain, took upon themselves a code of law as a testament to their love and fear of this G-d. After years of pondering this question, I eventually moved from a quite firm conviction of the latter to a far firmer conviction of the former, mostly as a result of newfound humility through learning. In any case, the upshot is the same--and arguably, subtly, the cause is the same, too. If one believes that a single G-d moves the world, including its humans, then one believes that the Law began as a reciprocal covenant between humans and G-d. 

Though it seems most difficult at first blush, it is in fact very easy to see and believe that the mesorah--the interpretation of Jewish law developed across rabbinical history until today--is true to the original sense of the Law. One comes to this belief by learning about the tradition of transmission and interpretation itself. The amount of rigor, devotion, and intelligence that went into this work--and still go into it--is staggering. The number of whole lives of whole men, to say nothing of man-hours, dedicated exclusively to this effort boggles the mind. The idea that one, in a few hours or a few lifetimes of skepticism, could outthink these geniuses, could outdo their combined work, is folly and idiocy (--not unlike, I might add, the folly and idiocy of some half-baked religious criticisms of the mesorah of science). The only defense of that armchair endeavor is that the human soul retains its sovereignty, and the human spirit its power of self-determination, in spite of any weight of evidence or practical calculation. And with that I turn to subjective truth. 

Maybe the Torah is not True. What, after all, is Truth? We have learned from *The Matrix* and Descartes that existence itself may be a sham. Perhaps we are only dreaming, or are in someone else's dream. And even if, big if, we really exist, we are only humans, who might as well be ants in our ability to assess life and to "understand G-d." What gives? How could we possibly decide from this fragile standpoint that the books in our library represent the sum total of G-d's Truth? 

We can't. And this fact is not lost on Jews. But, all the same, we know that Torah is true *to us.* If we are ants, then the limit of our brains is the space between the thorax and the mandibles. And yet we live in this space. This space is also the limit of our existence. Just as we will never be able to see or travel farther than the speed of light can do in one lifetime, we will never be able to think, or conceive, or read, or believe farther than this space will let us.

And so we make do, in the meantime finding what appeals to our little brain's finest senses of justice, reason, rightness, and meaning. We see what has been true for the best humans, from Moshe Rabbeinu down to our beloved grandparents. We see if it seems to be true for us. And now and then we do our limited best to turn and face G-d ourselves, and see if we can discern an answer about how best to serve Him. We have no choice: the day is short. The task will not be completed; but, our Fathers tell us, we must begin it somehow.

Torah's dignified, subtle, and intricate philosophy of personal and communal obligation, duty, and justice, as well as its stringent insistence on G-d's singleness, are the working through in human dimensions of extremely basic, and yet extremely innovative principles. These are easily taken for granted by those who have Western civilization as their birthright, but they were really at one time revolutionary, and would be so again if we lost them, ch"v. And thus, although neither the reflexive truth, the genetic truth, the referential truth, nor the subjective truth of Torah is alone dispositive, taken together, they form an image that, if not enough to kill for, is enough on which to base life and lives. 

B"H