How are we mere mortals to know which one is the correct one, if the real God won't tell us? It's all too silly for me.
How come God didn't tell us which one is right? if your someone who like Jews, Muslims, Christians, Hindus...etc who believe in the concept of "Almighty God" then you would realize he said that. In every old scriptures the message is same, it's to worship God and none beside him and not denying his existence. God clearly say it in the Quran and showed us the two way, the way of evil and the way of god and then he gave us the free will to choose which one we want to follow.
Suppose now we all know (Jews, Muslims, Christians...etc) tomorrow we have exam in History (Day of Judgment, Hell & Heaven). The Teacher told us listen my students, if you want to pass the history test read section ABC and ignore section XYZ, now those who listened and followed the teacher instructions would surely pass but those students who did the opposite for sure wouldn't pass even though they know that tomorrow's exam is History. This is the difference between we Muslims and others.
We understood the God's message clearly without changing any bit of his words, it's to worship him alone, but some students ignored the clear message and instructions of their god and followed instructions of other students who doesn't have the full knowledge about the contents of the test.
WE have created man from a mingled sperm-drop that WE might try him; so WE made him hearing, seeing. WE have shown him the way, whether he be grateful or ungrateful.
How we know, in addition to god's instructions use your common sense, can god's words contradicts itself? of course no, so if any book that contradicts it's own words ignore it and follow a book that cannot contradict itself. If that's not enough then test the one who is giving you the instruction, can God rest, sleep...etc? can almighty god get slapped or insulted? common sense would answer you NO WAY! because common sense have fixed attributes of God! He is almighty, all wise, all knowing, there is nothing like him....etc you would surely realize all false gods would fail in the test.