MANI - LDS missionaries ask investigators to pray about the book of mormon to get a burning in the bossom or an answer that it is "true".
What do you mean by "Pray to see if it is true"? Even if one gains a feeling or answer through prayer that its true, why do Mormons consider this enough evidence to believe in anything and everything Mormon?

JOEL - Mormons do not consider that enough evidence. That is not what I have learned in our Church. Prayer is only one of the important ingredients. Our scriptures tell us:

"Behold, I would exhort you that when ye shall read these things, if it be wisdom in God that ye should read them, that ye would remember how merciful the Lord hath been unto the children of men, from the creation of Adam even down until the time that ye shall receive these things, and ponder it in your hearts.
And when ye shall receive these things, I would exhort you that ye would ask God, the Eternal Father, in the name of Christ, if these things are not true; and if ye shall ask with a sincere heart, with real intent, having faith in Christ, he will manifest the truth of it unto you, by the power of the Holy Ghost.
And by the power of the Holy Ghost ye may know the truth of all things." (Moroni 10:3-5)

First we need to inform ourselves of the truth by reading the scriptures. Then we need to ponder, or study about what we have read or learned, and think it through in our minds. We also need to have faith in Christ when we pray(James 1:5-6); and of course we need to pray, expecting an answer through the Holy Ghost. The Holy Ghost is the testifier of all truth and we can't get His confirmation about the truthfulness of something unless we pray about it.

"But God hath revealed them unto us by his Spirit: for the Spirit searcheth all things, yea, the deep things of God.
For what man knoweth the things of a man, save the spirit of man which is in him? even so the things of God knoweth no man, but the Spirit of God.
Now we have received, not the spirit of the world, but the spirit which is of God; that we might know the things that are freely given to us of God.
Which things also we speak, not in the words which man’s wisdom teacheth, but which the Holy Ghost teacheth; comparing spiritual things with spiritual." (1 Cor 2:11-13)

We also must try to put into practice what we have learned, to see if it is true doctrine.

"If any man will do his will, he shall know of the doctrine, whether it be of God, or whether I speak of myself." (John 7:17)

For example, the scriptures tell us that we should pay our tithing.
"Bring ye all the tithes into the storehouse, that there may be meat in mine house, and prove me now herewith, saith the LORD of hosts, if I will not open you the windows of heaven, and pour you out a blessing, that there shall not be room enough to receive it." (Mal. 3:10, See also Ether 12:6)

We will realize the truthfulness of the principle of tithing after we do it and observe the blessings that come from it.
This probably needs to be done with almost every commandment we learn about before we can really know it is a true principle.
The Bible is one of our scriptures that we base our doctrine on and so we believe everything that it tells us about how to know the truth. Our missionaries teach all these things to their investigators including the importance of prayer. They may emphasize prayer a little more because they know that the Holy Ghost is the greatest and most important source of a witness for the truth of something.