This is my understanding on Mohammad. Realize that I am no expert and you should research on your own if you wish. My apologies if anything is wrong, but my focus is on understanding Jesus, not Mohammad.
Mohammad was kicked out of Meca, the 'capital city' of Islam. And moved to Medina. From there, he carried out raids on caravans and stole and murdered. At that time Meca was a city of many idols and gods. He decided that there should be only one god and decided on the Christian and Jewish God. He and his band of followers then returned to Meca violently and declared Allah as the one and only god. He was expecting support maybe from the Christians and Jews, but in light of his murderous rampage on the caravans the Jews and Christians rejected him as not of God.
I believe his actions speak for themselves. Is he from God? Unlike Saul, Mohammad was still killing and stealing after his declaration that he was a prophet. There was no change in him like in Saul. And David had a chance to kill his enemy, but didn't because it was not the time God had set forth for it. Did Christians and Jews kill? Yes, but under the umbrella of war, and under the guidance of God. Also those who were killing before they had faith changed. Muhammad didn't change, he was more of a pirate with a following than anything else. Try to realize that somewhere around 70% of the people who call themselves Muslims can't even read the Quran. Then learn only by what a teacher says, regardless if it is even written.
Be blessed,
Nigh