One of the most interesting and useful resources on the internet is the Catholic Encyclopedia. Here is a wonderful article from the anceint book "Shepherd of Hermas" about spotting the difference between a false and true prophet of God:
The Shepherd of Hermas (Book II, Commandment 11)
"He pointed out to me some men sitting on a seat, and one man sitting on a chair. And he says to me, "Do you see the persons sitting on the seat?"
"I do, sir," said I.
"These," says he, "are the faithful, and he who sits on the chair is a false prophet, ruining the minds of the servants of God. It is the doubters, not the faithful, that he ruins. These doubters then go to him as to a soothsayer, and inquire of him what will happen to them; and he, the false prophet, not having the power of a Divine Spirit in him, answers them according to their inquiries, and according to their wicked desires, and fills their souls with expectations, according to their own wishes. For being himself empty, he gives empty answers to empty inquirers; for every answer is made to the emptiness of man.
"Some true words he does occasionally utter; for the devil fills him with his own spirit, in the hope that he may be able to overcome some of the righteous. As many, then, as are strong in the faith of the Lord, and are clothed with truth, have no connection with such spirits, but keep away from them; but as many as are of doubtful minds and frequently repent, betake themselves to soothsaying, even as the heathen, and bring greater sin upon themselves by their idolatry. For he who inquires of a false prophet in regard to any action is an idolater, and devoid of the truth, and foolish.
"For no spirit given by God requires to be asked; but such a spirit having the power of Divinity speaks all things of itself, for it proceeds from above from the power of the Divine Spirit. But the spirit which is asked and speaks according to the desires of men is earthly, light, and powerless, and it is altogether silent if it is not questioned."
"How then, sir," say I, "will a man know which of them is the prophet, and which the false prophet?"
"I will tell you," says he, "about both the prophets, and then you can try the true and the false prophet according to my directions.
"Try the man who has the Divine Spirit by his life. First, he who has the Divine Spirit proceeding from above is meek, and peaceable, and humble, and refrains from, all iniquity and the vain desire of this world, and contents himself with fewer wants than those of other men, and when asked he makes no reply; nor does he speak privately, nor when man wishes the spirit to speak does the Holy Spirit speak, but it speaks only when God wishes it to speak.
"When, then, a man having the Divine Spirit comes into an assembly of righteous men who have faith in the Divine Spirit, and this assembly of men offers up prayer to God, then the angel of the prophetic Spirit, who is destined for him, fills the man; and the man being filled with the Holy Spirit, speaks to the multitude as the Lord wishes. Thus, then, will the Spirit of Divinity become manifest. Whatever power therefore comes from the Spirit of Divinity belongs to the Lord.
"Hear, then," says he, "in regard to the spirit which is earthly, and empty, and powerless, and foolish.
"First, the man who seems to have the Spirit exalts himself, and wishes to have the first seat, and is bold, and impudent, and talkative, and lives in the midst of many luxuries and many other delusions, and takes rewards for his prophecy; and if he does not receive rewards, he does not prophesy. Can, then, the Divine Spirit take rewards and prophesy? It is not possible that the prophet of God should do this, but prophets of this character are possessed by an earthly spirit.
"Then it never approaches an assembly of righteous men, but shuns them. And it associates with doubters and the vain, and prophesies to them in a comer, and deceives them, speaking to them, according to their desires, mere empty words: for they are empty to whom it gives its answers. For the empty vessel, when placed along with the empty, is not Crashed, but they correspond to each other. When, therefore, it comes into an assembly of righteous men who have a Spirit of Divinity, and they offer up prayer, that man is made empty, and the earthly spirit flees from him through fear, and that man is made dumb, and is entirely crashed, being unable to speak.
"For if you pack closely a storehouse with wine or oil, and put an empty jar in the midst of the vessels of wine or oil, you will find that jar empty as when you placed it, if you should wish to clear the storehouse. So also the empty prophets, when they come to the spirits of the righteous, are found [on leaving] to be such as they were when they came.
"This, then, is the mode of life of both prophets. Try by his deeds and his life the man who says that he is inspired. But as for you, trust the Spirit which comes from God, and has power; but the spirit which is earthly and empty trust not at all, for there is no power in it: it comes from the devil.
"Hear, then, the parable which I am to tell you. Take a stone, and throw it to the sky, and see if you can touch it. Or again, take a squirt of water and squirt into the sky, and see if you can penetrate the sky."
"How, sir," say I, "can these things take place? for both of them are impossible."
"As these things," says he, "are impossible, so also are the earthly spirits powerless and pithless. But look, on the other hand, at the power which comes from above. Hail is of the size of a very small grain, yet when it falls on a man's head how much annoyance it gives him! Or, again, take the drop which falls from a pitcher to the ground, and yet it hollows a stone. You see, then, that the smallest things coming from above have great power when they fall upon the earth. Thus also is the Divine Spirit, which comes from above, powerful. Trust, then, that Spirit, but have nothing to do with the other."