True and false prophets

2 Chronicle 18:1-11

Now Jehoshaphat had great wealth and honor, and he allied himself with Ahab by marriage. But Jehoshaphat also said to the king of Israel, “First seek the counsel of theLord.”

But Jehoshaphat asked, “Is there no longer a prophet of the Lord here whom we can inquire of?”

The king of Israel answered Jehoshaphat, “There is still one prophet through whom we can inquire of the Lord, but I hate him because he never prophesies anything good about me, but always bad. He is Micaiah son of Imlah.”

“The king should not say such a thing,” 

All the other prophets were prophesying the same thing. “Attack Ramoth Gilead and be victorious,” they said, “for the Lord will give it into the king’s hand.”
Jehoshaphat's son Jehora marriaed Athaliah, Ahab's daughter. Far from reuniting the kingdom, this link brought near-disaster to Judah in later year.
Why?
A prophet of the Lord say the truth of God, he won't say the words to please men.
A good man should not have too much relationship with the evil one, but for sake of gospel we have to do so. But don't follow them. Blessing of the Lord is always with his sons.
Ture and flse prophets:
It was never easy to distinguish between the two. Jehoshaphat senses from the shallow optimism of their message that these prophets are merely telling Ahab what he wants to hear.

• The false and the true can only be distinquished by their life and message, not by methods or manner. 
• No true prophet made a prediction which failed to happen; practised or encouraged immorality; or led people away from God and his law.

How?
A follower of the Lord seek the truth, God's word are the truth