The proof that Prophet Mohammed was an apostle from God is cleared by following points: 
1- The glad tidings narrated about him in torah and the gospel after prophets Moses and Jesus. Abdullah Ibn-Amr Ibn-Al-Aas read and perused the previous scriptures, he said that the prophet was described in the torah and the gospel the same as he was described in the Koran. 
Then he mentioned the text he found in them: (O Prophet! truly We have sent you as a witness, a bearer of glad tidings, and a Warner, you are My servant and My apostle, I called you Al Motawakel (the one who put his trust in God in everything), he is not severe or harsh or roaring in markets, he doesn’t repel evil by evil but he forgives and pardons, I won’t end his life until I reform by him the crooked religion, so that they say La Ilaha Illa Allah (There is no God but Allah), and by him I open blind eyes and deaf ears) . There are so many texts narrated about that, whoever wants to see more he should go to the books mentioned that and they are so many.
2- What Heracles the Roman emperor mentioned in the dialogue between him and Abu-Sofyan, when the prophet sent him a message calling him to Islam, it said: (In the name of God the most gracious the most merciful, from Mohammed the apostle of God to Heracles the emperor of the Romans, be a Muslim and you will be safe, God will double you your favor, if you shun then you are responsible for your followers). 
The Koran says: 
3:64. “Say: "O People of the Book! Come to common terms as between us and you, that we all worship none but God, that we associate no partners with him; that we erect not from among ourselves Lords and patrons other than God." If then they turn back, say you: "Bear witness that we (at least) are Muslims (bowing( to God’s Will.” 
When he got this message he asked his followers if there were any of his family in Jerusalem, they searched its markets and found Abu-Sofyan Ibn-Harb with few people from Mecca, and Abu- Sofyan was the leader of the people of Mecca and their leader in the fight against the apostle of God. Heracles asked them to enter and asked about their leader, they pointed to Abu-Sofyan, he told them to stand behind him and pointed him as a liar if he said anything untrue, then he asked him through a translator: 
Heraclius: Has anyone of his family alleged this thing before? Abu-Sofyan: No. 
Heraclius: Has his family lost a reign or dominion and he seeks it back? Abu-Sofyan: No. 
Heraclius: What is his ancestry? 
Abu-Sofyan: He is moderate (middle class) amongst us. 
Heraclius: Was he a liar amongst you? Abu-Sofyan: No. 
Heraclius: Who follow him? The rich or the poor?  
Abu-Sofyan: The poor. 
Heraclius: Do his followers increase or decrease? 
Abu-Sofyan: They increase., Heraclius: Does any of them return and leave him after following him? 
Abu-Sofyan: No., ....…………….. Etc. 
Then Heraclius said: You told me that he never lied to you, so he never to let lying to you and lie to God, you said that his followers are the poor and those are the followers of prophets, you said that no one of his followers leaves him and this is the truth when it touches hearts and you said that there was not reign in his family so he seeks it back. 
If you are truthful then this is the prophet Jesus mentioned to us, lest the people around me, I would go and wash his feet and God will support him until he owns what is below my feet. 
3- What was narrated after priests, rabbis and monks who were alive when he was born or they told about him before he was born and this is mentioned a lot in books and references talked about him (blessing and peace of God be upon him). 
4- What modern technology proved about the miraculous nature of his words and his phrases, and also what is mentioned in the holy Koran as both Koran and Sunna agree with the latest theories and scientific rules unlike torah and gospel because they not agree with true modern science. This was mentioned in many researches and books such as: "The Bible, The Qur'an and Science by Morris Bokay". 
5- Abstaining from everything the human nature seeks such as, desires, wishes, position, honor, food, drinks, clothing, residence, vehicle, women, etc, though he needs such things badly, only a prophet’s soul can give up such things. 
6- His great effect upon those around him, he formed a perfect talented group for the humanity in all fields and all ages. History has never seen a true love from one person to another as Abu-Bakr loved him (Blessings and peace of God be upon him). There has never been in history a just and fair ruler as Omar. Genius in leading armies gets humble if compared with Khalid Ibn-Al-Waleed. It's so rare among people to find intuition in fatwa as that of Imam Ali Ibn-Abi-Taleb. There are so many of those perfect followers of the prophet in all fields this is admitted by enemies before friends. 
7- Throughout history, there has never been a superpower or an empire but they depend on moral values and human principles mentioned in his call even if they not taking this directly from him. 
We see modern civilization depends mainly on the values he calls for such as hard work, sincerity, truthfulness in work, giving more attention to time and other good values, this shows that what he gets is suitable and useful for humanity at all times and everywhere. 
As he has never been educated at universities or scientific institutions nor accompanied any philosophers or scholars, so this was the true evidence that what he got was just a As he has never been educated at universities or scientific institutions nor accompanied any philosophers or scholars, so this was the true evidence that what he got was just a revelation from God and that he is a prophet from God.

  Book questions from non-Muslims