It is actually Moroni 10:4 that says that. If you reference Alma 32, which I listed first for this reason, then you should note vs. 21 and 26-28 from which you should be able to recognize that being willing to follow the advice seriously is sufficient. Also, the whole think about it and then pray about it is a pattern (as Moroni 10:5 notes) that can be used for anything. Therefore, in your case, I would probably ask about God or Christ before asking about the truthfulness of the Book of Mormon.
Moroni 10:3 tells me I need to have faith in Christ before I ask for a sign. That kind of defeats the purpose.
It is actually Moroni 10:4 that says that. If you reference Alma 32, which I listed first for this reason, then you should note vs. 21 and 26-28 from which you should be able to recognize that being willing to follow the advice seriously is sufficient. Also, the whole think about it and then pray about it is a pattern (as Moroni 10:5 notes) that can be used for anything. Therefore, in your case, I would probably ask about God or Christ before asking about the truthfulness of the Book of Mormon.