I will let you decide for yourself what the definition of a True Messenger is, but the truth is easy to define. It is knowledge of things as they are, and as they were, and as they are to come. How would this sort of knowledge affect you?
What if Socrates spoke with you? What if Siddhartha presented an idea to you? What if Zarathustra was trying to explain something? Would you recognize greatness in the words and concepts of Confucius? What if John the Beloved was explaining the history of the world from the beginning to the end? What if Jesus Christ was speaking on a mountainside? These are all ancient men though, so what if an uneducated 20-year-old farm boy from New York and whom could hardly read wrote and published a 500+ page book based on the Bible narrative and is responsible for bringing forth the most wealthy, per capita, religion in the world?
Would you recognize a person whose brain and understanding had been enhanced through DNA manipulation or whatever you want to call it by their outward appearance? By the the people he mingled with? By the way he dressed? What if the words or histories attributed to ancient True Messengers weren't words or deeds spoken or performed by them, but they were made up by men with ulterior motives to protect their wealth and power? Does that make them any less of a True Messenger? How would you know the truth regarding these things unless a modern True Messenger revealed them to you? Or are you smart enough to figure it out all by yourself?
It's easy to say yes, you would recognize and accept these ancient ones, but you weren't there for any of them. Or maybe you don't believe a person's brain and/or understanding can be enhanced after they are born and grown.
What about Joseph Smith, the man who claimed he was a True Messenger sent to America? What if he really was a True Messenger? If he was, then he is the one most relevant to you, but maybe you are biased against him due to some of the things he said or did that are not congruent with your current belief system. What if he had a true message, but early on the people rejected it, so he had to change his plan to present a softer easier message to appeal to them more and in so doing he said or did many things that were not easy to understand or caused people to reject him outright? What if he did those hard-to-understand things because if he spoke or acted in plainness then the people would have killed him, which the people eventually did anyway? What if he did not practice polygamy? What if he did not believe in religion or have a desire to start one? What if he allowed the people to have "priesthood authority" (which meant nothing to him) because they wanted it, and continued to give the people what they wanted in order to extend his life so he could drop subtle hints for another fifteen years as to what the real truth was? What if he was compelled by the religious beliefs of early Americans to utilize Biblical words and concepts to express truth and now the result of his efforts is similar to the results of the efforts of Muhammad, another possible True Messenger whom is considered to be the founder of the religion of Islam?
What if Muhammad did not say or do anything that the Quran claims he did? What if the religion of Mormonism and Islam are both false, but their founders were True Messengers? What if the religions that stemmed from them represent nothing of the truths they were really trying to convey? What if all the religions that derive from Abraham (Judaism, Islam and Christianity) are all based on a mythological figure?
Aren't truths like these important to know? Maybe you're smarter than me and can explain in detail how the world has come to the place where it is. Maybe you know all this stuff already.
If you don't accept religion, that is good. Religion, especially the ones that are linked to Abraham, is guilty of bringing the worst form of human ugliness upon mankind. However, religions contain followers who are the most proud people of all people. Therefore, if a True Messenger had a message for the people of earth, wouldn't it make sense for a True Messenger to be sent to the most proud religious people at that time? And if that makes sense, then wouldn't the True Messenger have a better chance of conveying a message and the people accepting it if the True Messenger used words, phrases, symbolism, metaphors, analogies, etc. that the people are most familiar with?
Is that what Jesus did? Is it what Joseph Smith did? Is it what John the Beloved did?
For example, what if John the Beloved, whom is believed to be the author of the Book of Revelation in the Bible, was not religious at all? What if he didn't believe in the Jewish god or Christian god or in any god, except himself, but in order to convey a true message about humanity and have it be inserted into the most influential book the world has ever known, the Bible, he had to write it in such a way that it used imagery and symbolism from the culture from which he came?
Or, to put it another way, what if Socrates presented ideas designed for the ancient Greeks to the people in China using Greek concepts, myths and legends? It wouldn't work. So a True Messenger's message is always hindered by the societal norms and culture of the people.
But, what if the message from True Messengers is always the same making it easier for the masses to recognize a True Messenger? But because the message is masked in the rhetoric of the culture in which the people live, the real truth has to be sincerely searched for, not merely stumbled upon, so to form an opinion for or against a True Messenger based on cultural, religious and/or political biases would prevent you from recognizing the truth of all things. Is this what you do?
If the True Messenger is just a messenger then you might ask "from whom?" would the message come. What if the message from a True Messenger was from you, yourself? Would you want to know what yourself is trying to tell you? Or would you simply say that sort of stuff is hogwash that reeks of spiritualism, voodoo, fairy-dust bullshit?
It is your choice what you want to believe.