Your take on the difficulties associated with false prophets, "fruits," God's will, and vigilance strikes a chord with me. At times I rather despair of our being able to discern false prophets. We have trouble enough sorting through our own self-deceptions! Your closing lament (if you will grant me the term) captures the contemporary challenge: "I don't know how we can protect ourselves against false prophets...there are prophets of one faith or sect who argue against those of another faith or sect but there is no way to tell which is the truer."
Your words remind me of an episode in the novel The Two Towers. Aragorn encounters a young nobleman who is confused and frightened by the competing claims of the time. The nobleman (I'm paraphrasing, not having the novel to hand)cries out: "How is a man to choose in such times as these?" Aragorn's reply (again, paraphrased from memory) braces him: "As he has ever chosen. Good and evil have not changed in a day."
The key point is: Nothing absolves us from the burden or responsibility of choosing. So, when it comes to false prophets, truth, right, wrong, and the like I may be confused but still I must choose, either deliberately or by default.
Given that cheery thought, here are the guidelines I follow. I draw on the Christian tradition, mix in a dash of what I hope is wisdom gleaned from experience, and leaven the whole thing with intentional humility (i.e. acknowledging that I will make mistakes and resolving to try to admit mistakes when they happen and rectify them as I can). Here are my guidelines.
(1) Does the "prophet," teaching, or proposed action square with persons being created in the image of God? If I follow up, will I find myself according others the freedom and responsiblity and dignity inherent in their being made in the image of God? It's surprising how many times this simple guideline leads me to choose not to enlist in a given "prophet's" agenda.
(2) Would I want to be on the receiving end of a given prophet's agenda? If not, I become very cautious about embracing such agendas.
(3) Will the agenda, teaching or action contribute to the enhancement of human community or instead farther fracture it? If division seems to be the likely result, I grow cautious.
(4) Does the agenda, teaching, or action require some genuine sacrifice on my part, or does it mostly serve to protect my interests (economic, career, comfort zone, etc.). I tend to be suspicious of anything that essentially promises to help me "keep what's mine."
Obviously, my decision making is more complicated than such a simple list implies, but still I find the four guidelines help challenge me to take seriously the matter of choice.
Equally obviously, I've not answered the common question: "But how can you know what is true and take actions accordingly." I'm not sure that's the best question, frankly, even if it does appeal to folk reared in the scientific era. Instead, I think aspiring to guide my actions on the basis of seeing God in others, putting myself in the other person's shoes, building community, and embracing sacrifice for the sake of others offers a more productive approach. Mind you, my approach does not provide surety of any kind! But it does preserve me from paralysis.