Matthew 7:15 warns us to "Beware of false prophets, which come to you in sheep's clothing, but inwardly are revening wolves." 
 
The next verse, however, provides additional councel. "Ye shall know them by their fruits." 
 
The fact that we should beware of false prophets implies that there are also true prophets. 
 
We can tell the difference between them by their fruits. As for the prophet Joseph Smith, his principle "fruit" was the Book of Mormon. If this book is true (i.e. a good fruit), he was a true prophet. If it is not, then Smith was, as Matthew put it, a ravening wolf in sheeps clothing. It is up to each of us to taste that fruit and find out for ourselves if it is good. 
 
The Book of Mormon invites all to read, ponder and ask God Himself whether it is true. God promises that when we ask sincerely and with faith, He will answer our prayers through the power of the Holy Ghost.

Mormons have prophets because "we beleive in the orginization of the primitave church" (Articles of Faith 6). 
Why would the orginization change over time?

Yup. 
 
False Prophets = Bad 
 
True Prophets = Talks with God =Good  
 
Listen and follow what the True Prophets tell you! Disregard the False ones. : )

the scriptures(bible) also tell us that we were given some prophets for our good, but do what has already been stated, test his fruit, it is good. i testify that Joseph Smith was a prophet of god, he did receive revelation for our day and he was guided by god to do the things that he did.

Could you please tell what the standard is for determining a bad vs. a good prophet? Please be specific and cite your reference.

Personally I think a bad prophet is an oxymoron. A bad prophet by definition is wicked. I don't believe that true prophets of God can be wicked. My idea of a prophet is someone who is God's prophet on the earth and speaks for Him. God only will use righteous people to represent him. Therefore I don't think there are bad prophets. There might be wicked people who claim to be prophets, but in my mind they are not prophets - just evil and wicked.

Yes, he did, but how does one tell a false prophet from a true prophet? 
 
Matthew 7:16 says "Ye shall know them by there fruits" 
 
Look at the fruits of Joseph Smith are the Book or Mormon and the Church of Jesus Christ of Latter-day Saints.  
 
Look to our Church and the Book of Mormon to decide if the fruits of this Prophet show him to be a true prophet or a false prophet.

Aren't false prophets tricky? I mean couldn't you follow someone who seems to be doing the right thing, and therefore your life have more meaning. Its hard to know if God is really answering you, or if your head is telling you he is. How can you tell the difference between what you feel is right and what is right?

Tina, it seems a lot trickier then it actually is. The more you think about it and question and wonder the trickier it gets.  
As Art put it in the first comment "It is up to each of us to taste that fruit and find out for ourselves if it is good." 
That doesn't necessarily mean blindly following and seeing if good things come of the work, and simply judging of a true prophet. It states we need to find out for ourselves if it is good. We can do that by asking. 
 
Many members of the church have a testimony that Thomas S. Monson is the true prophet on the earth today, because we humbly knelt on our knees, studied the scriptures and asked out heavenly father to bear witness to us that he is the true prophet today. 
 
Tina, feelings are extremely hard to explain. Its almost impossible for any of us to tell you how to feel the difference between what your asking, but we do know that you can FEEL the difference.  
 
If you truly want to know, do what we suggest. Read the book of Mormon, and pray to know if its true or not. Then you will know the difference. I hope this helped Tina.

In Christianity a prophet is one inspired by God through the Holy Spirit to deliver a message for a specific purpose. It is often associated with predicting future events, but in biblical terms it is wider and can include those given the power to preach repentance to those who do not want to hear the message and to warn of God's wrath for disobedience. God's calling as a prophet is not considered to elevate an individual for their glory, but for the glory of God and to turn people to him. Some Christian denominations would limit that and exclude those who receive a personal message not intended for the body of believers, but in the Bible on a number of occasions prophets were called to deliver personal messages.[9] The reception of a message is termed revelation and the delivery of the message is termed prophecy. 
Anyone who claims to speak God's words or teach in his name and is not a prophet the Bible terms a false prophet. One test given in the Old Testament in Deuteronomy[10] contains a warning of those who prophecy events which do not come to pass and said they should be put to death. Elsewhere a false prophet may be someone who is purposely trying to deceive, is delusional, under the influence of Satan or is speaking from his own spirit