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.