IIRC, if a prophet says a [small] prophecy and it comes true we should believe him.
What magnitude of a prophecy should it be - war is coming the next week, there's gonna be a solar eclipse next month, X is gonna win the elections, tomorrow is going to be sunny with a chance of precipitation?
Sorry, I don't remember the right sources (please help)
This is elaborated in Mishneh Torah, Hilchot Yesodei HaTorah, Chapters 7 and 8.
The primary idea is that the Prophet is of outstanding character and meets all the pre-requisites listed by Rambam. Interestingly enough, Rambam mentions that if a Prophet is sent to communicate to someone or a group, it is the intended recipient who is to judge the veracity of the Prophet.
Rambam says that the Prophet will be given a sign(s) or miracle that will be meaningful to them (those intended to receive the message) and that he will then communicate his message. If they recognize him to have fulfilled all the pre-requisites for being a valid Prophet and they are convinced by his sign(s) or miracle, then they are required to listen to his message, however big or small it may be and whatever subject it is about. That initial message, which demonstrates him or her to be a true Prophet of G-d, must be 100% true, meaning it must unfold exactly as the Prophet states.
Elsewhere (Chapter 10), Rambam explains that a prophecy can be either negative or positive. Those which are positive prophecies (meaning for the good) must happen. They cannot be overturned or rescinded. Negative prophecies however, can be overturned through teshuvah.
This would seem to imply that the initial prophecy which proves credentials would never be negative.
To determine wether one is a prophet we must first define our terms. What exactly is a prophet? In essence, prophecy is solely based on the acquired intellect. Prophecy, according to the Rambam is specified by intelligence. Knowledge is key and is, therefore, the pre-request for prophecy. This includes the investigation into metaphysics, philosophy, and science. Those are the credentials for prophethood.
Accordingly, when emanation from the overflow of Active (or Agent) Intellect enters a person's rational faculty and imaginative faculty, alluding a person's reasoning abilities to foresee the future, a prediction is made. For prediction is a manifestation of intelligence. But this is not entirely true for a prophetic vision may be expunged or canceled. Hence a prophet can only see what ought to be and not what will be. That is to say, when prophecy is revoked, it is not the cause of G-d but the nature of the person's continuity at work. Thus prophecy is a natural event. Which is to say, though intellectually gifted, a healthy-wise person may befall dire circumstances that prevent him or her from prophecy, say if the person becomes sick or ill, preventing them to think. When such a divergence is inevitable, the prophecy may be lost, if not compromised.
But it is more than the cold calculated intellect at work. Such a person must be moral in all aspects of life if they wish to become immune to natural law. The Rambam states that a prophet is a moral person who loves G-d yet "love" is an emotion and the Torah cannot ascribe a feeling to G-d. When the Torah commands us to love G-d it is expecting us to put that "love" in the form of application. That is, in accordance with right action, right speech, and right thought, one should study G-d's Creations. This is what it is meant when the Midrash says Abraham discovered G-d by studying the heavens, natural law, G-d's laws. This is similar to Plato's Republic, who explains the perfect lawgiver, a Philosopher-King, who has reached ultimate perfection. A perfection which Maimonides attributes to a prophet.
Thus the qualities of a prophet, Maimonides affirms, is not secluded to Israel alone but is entitled to everyone. Even non-Jews can become prophets. Indeed, Maimonides considered Aristotle to be a prophet because of his immense intelligence. Thus a prophet is a person who loves G-d, develops his or her intellect, and can use midrashic imagination to carry out G-d's will.
If a person does not meet these requirements as described above, you do not have to listen to him.
