Discussing Mormonism: How would you reply?

Here's a question I have for everyone...

When confronted with significant evidence opposing Mormonism, or when they don't know what else to say, Mormons will often say, "I believe the Book of Mormon is true because I prayed about it and received a peace about it." Then they would often urge me or even ask me to promise to pray and ask God if the Book of Mormon ("BoM") is true. They will tell and re-tell their "testimony" about how much they believe the BoM is true. How would you respond to that?

Side note: From what I understand, the BoM isn't even really central to their religious practices--the Doctrine and Covenants ("D&C") is more central because it lists the do's and don't's. However, the D&C has a lot of things in it that are very problematic so they don't pass out D&C's. You can find it online though.