Yes it is harder. A prophet or teacher who works within an established religion has that going for him as you stated.I hear what you are saying and I agree with you, but only up to a certain point. There is no Jew in the world that would have trusted the Maggid had he instituted a new religion with a new book. Heck, even if he didn't make a new religion, but claimed to be a Jewish prophet, he'd probably have been lynched.
And this case of Muhammad is even harder. In the case of the Maggid, there was already an existing framework within which a Jew could ascertain that the Maggid is coming from a real concept. But the Arabs of the day didn't even have that. A random man, they had no relationship to, gets up and claims to be a prophet of G-d and builds them an entirely new religion from scratch. They don't even have a framework to determine if what he is teaching them is called serving G-d better. And before you argue that a framework isn't necessary, consider that he gave them a framework. Which means both he and they now believe that a framework is necessary. So what standard did they have to decide he was teaching them this?
The test is necessarily more severe for what some call a World Teacher or Avatar and in the case of this thread Muhammad. We've seen many false gurus or prophets in the past few decades who have deceived many and in some cases fatally so. So it's dangerous to put one's life and one's future in unclean hands.
Does the person live their words? How do they treat people? Does their lifestyle betray lust, greed or other desires or do they live positive values? How do their followers behave during the years when the prophet lived. Every religion has those who pervert the message and today Islam is in particularly a hard place because of the terrorists who claim to be Muslims. But when I was studying world religions, I found this story. Whether or not it's true is not important. The message of this story is to me the point and reflects that people need to understand their emotions in deciding whether or not someone is a true prophet:
At the Battle of the Ditch, the noble Imam 'Ali had knocked an enemy soldier to the ground and was raising his sword to kill him, when the unbeliever spat in the blessed face of the Valiant Lion, may Allah be pleased with him. Imam 'Ali at once stood still and refrained from killing his enemy. Hardly able to believe his own eyes, the unbeliever asked: "Why have you spared me, O gracious one?"
To this, the noble 'Ali repliied: "Your property and your life have become sacrosanct to me. I am not authorized to slay you. I can receive permission to kill only in holy combat, in fighting commanded by Allah. Just a few moments ago, I had overcome you in battle, knocked you to the ground and was on the point of slaying you. But when you spat in my face, my selfish anger was aroused against you. If I had killed you, I would have slain you not for Allah's sake but for my own selfish reason; they would then have called me not a champion warrior, but a murderer. When you spat in my face, my selfish passion threatened to overwhelm me, so instead of striking you with the sword for my own sake I struck my passion for the sake of Allah, Exalted is He. There you have the reason for your escape."
Another test to me is what someone says and writes. In this case the Quran. I'm far from a scholar of Islam, but I've noted how little most Muslims know what is in the Quran and how others often cite passages out of context and without considering the document as a whole. Of course, translation is a tricky business especially with Quranic Arabic, so I often rely on looking at multiple translations. But in writing this post, I stumbled on a web page that talks about the difference between how the Quran refers to the "Children of Israel" and "Jews". To us, there is no distinction but http://www.eretzyisroel.org/~jkatz/quranhadith.html draws a historical distinction which to me is meaningful. And noting 17:104, I looked up other translations http://submission.org/d/x/webqt.php which illustrates the problem of translation.