The problem with discerning whether a prophet’s prophecy has come true is that even true a prophet's prophecies may not come true (at that time) if the people repent as mentioned in Jeremiah 18. We also see an example of this in Jonah where his prophesy doesn’t come true due to the repentance of the Ninevites. The problem of discerning the truth or falsity of a prophecy based on the morality or worship of Y-H-V-H is demonstrated in the pagan prophet Balaam making a true prophecy (see Numbers) and pagan kings like Nebuchanezzar (see Daniel) and Pharoah (see story of Joseph in Genesis) having dreams that are considered true prophetic dreams.
I think the ultimate determinate of a true prophet or prophecy is whether the words of the prophet are leading to the service of proclaiming the glory of Israel’s God and/or the Torah as a guide to life. If they lead away from God’s glory and do not call others to holiness (kedushah) and repentance (teshuvah) then they are false prophets.
The concept of covenant and fidelity to the covenant is also part of this discernment of true or false prophets. The role of the covenant community and its tradition is important for the ongoing value of the messages of the prophets to successive generations. The prophets stress the priority of the ethical and moral dimensions of the covenant which is rooted in the love, justice, holiness and mercy. However we must be aware that, as Ratzinger taught, the Torah is a unity "...For, to be sure, one cannot simply separate out universally valid moral principles and transitory ritual and legal norms without destroying the Torah itself, which is something integral, which owes its existence to God's address to Israel. The idea that, on the one hand, there are pure morals which are reasonable and universal, and on the other that there are rites that are conditioned by time and ultimately dispensable mistakes entirely the inner structure of the five books of Moses. "The Decalogue" as the core of the work of the law shows clearly enough that the worship of God is completely indivisible from morals, cult and ethos..."
 Joseph Cardinal Ratzinger, Many Religions: One Covenant, (San Francisco: Ignatius Press, 1999), 21ff.