I am not a Muslim scholar however I will try to make my case: Why Quran is the word of God. There is a verse in Quran states that: "If it was not from God they would find a great difference". Let us here imagine that Quran was not the word of God and the messenger Muhammad (PBUH) made it up or it was inspired by Satan as some in the West believe. Let us examine what difference it will be if it was not from God:
1- Weakness: the words of the speaker of Quran is strong from its beginning to its end. If not from God will find like in human speech times of weakness and contraction. I challenge you to read any page and think if this is the power and glory of a God or a human who is talking to us.
2- Politics: There is no politics in Quran. It is God who created us speaking to all humans as his slaves. He is not praising a group of people except when they are following his commands. He is not mentioning names of famous Arabs or tribes to get their approval. His not mentioning Muhammad (PBUH) friends or family members and so forth.
3- Different commandments: almost the same ten commandments that when broken are considered major sins as it is in the bible. Same strong God as in the bible who revenge to his commandments (which are ultimately good for humans). I do not think Satan would tempt us to follow God's commandments.
4- Different Prophets: though God did not mention all the prophets in Quran he mentioned the most famous and those who suffered the most. He told their stories several times in the book. He praised them and according to their sufferings the 5 mentioned most were Noah, Abraham, Moses, Jesus and Muhammad (peace be on them all). So the prophets were different in their Sharia (religious laws) they carried the same core: worshipping God and morality. Quran did not create different prophets but same link with same chronological order.
5- Not Moses and pharaoh: God loves this story and he repeated over and over in the bible and the Quran ( 7 times). It basically tell you the common struggle between the believers and oppressed against the unbelievers and oppressors or tyrants. It is the story of most prophets in the bible and even in our political history. Pharaoh was a kind of believer he believed in himself and denied his creator. You love it when God turned pharaoh wickedness with a striking defeat. He let him too close to catch the Israelites then all of sudden he split the water and after they passed through it he drowned pharaoh and his soldiers. At that point pharaoh said "I believed on what Beno Israel believed on" but it was too late for him. The stories of the common prophets you read them and enjoy them in the Quran. Why someone would create all of these stories. Why he got them that right as if he was watching with his own eyes. Why specifically Moses and pharaoh unless he is the same God who enjoyed his glory against the injustice of the pharaoh so he went down to earth to teach every tyrant this big lesson.
6- Different good and evil: Same Good and evil struggle and same temptation of Satan to us as he did to Adam and Eve. Quran carried same story. I do not think Satan will inspire the Quran to warn us of himself and his temptations to us or ask us to worship and glorify God.
7- Playing politics with the Jews and Christians: Same message to Jews and Christians since Quran started. If Quran was not from God you will find a lot of politics to win the Jews and Christians. It was clear from the beginning God was angry with the Jews at times when they did not follow his prophets and God anger was also strong against the misguided Christians who took Jesus as his real son. There is no compromise or politics from the start. If it was not God's words you would find much politics. The real politics are to be played by Muslims themselves to try to have harmony with the Jews and Christians. It is how to win them and in same time you are not changing God's words. It is very simple if they still do no believe in the Quran as God's word we can wait for the Christ he will come in few years. With his glory as he descent from heaven all people around the world will believe in him. He will tell us all the truth. What Muslims are asking is for all of us to get righteous and prepare for his second coming.
8- Heaven and hell: As we do in our life with our children, with our employees and even between nations people use the carrot and stick. For God you would expect to have both of them great. Both were mentioned in the Quran with a lot of details. Any one would make that up and pretend that God said that you expect him to be the first who will fear hell. People who question God mercy should understand that the requirements to go to heaven are few and very good to all of us. If someone would tell me that we will beat you up everyday and you will go to heaven for eternity I will tell him I got a good deal.
9- Not criticizing Muhammad (PBUH): it does not make sense to me if the Quran was created by Muhammad to criticize himself. In the few times God corrected his messenger he made it clear to him it is a mistake and told him what should be the right. As an Example when he was preaching to the leaders of the Arab tribes and he avoided a blind poor man. God made it clear that this man does not come after them. This man name is Abn Maktoum and after that Muhammad (PBUH) would give him the best reception when he sees him telling him "you are the man God had blamed for". I do not think Muhammad (PBUH) would create a book and criticizes himself.
As God mentioned in Quran talking to Muhammad (PBUH) "and so we inspired you through our spirit (Gabriel) with our message, and before that you did not know what the book and what the faith, but we made it (the Quran) a light that guides who we would like from our slaves, and you are a guide to the straight path" Quran 42:52.