We can know because it has more original manuscripts then any other book in history and manuscripts that are with-in a couple of hundred years of the original. Where like the u'ran which has none or the sayings of the prophet in the hadith, that are proven wrong through science and common sense. Like the prophet said, "an believer eats with one intestine and a unbeliever eats with seven intestines" or "do not look up to the sky in prayer and supplication lest our eyes be snatched out". The story about the fish and the Jews is a real kicker. There's a village and the people fish the pond every day except the Sabbath, on the Sabbath the fish come up from the water and mock the people. So the people (Jews) decide to fish on the Sabbath, they catch fish and eat them. When they finish eating they turn into rats.
Do you think that's a holy book or the sayings of a holy prophet, the one that made me wonder if this is how we got the boogy man. "satan resides in the nose as you sleep" I not quoted any verbatim, but you get the idea. If someone is doutful I can quote verbatim and give the chapter and sura of where it is in the hadith.