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.