Mormons say they know that God exists and the Book of Mormon is true because they can feel God's presence, as requested by prayer.   But they are deluded.

Here are some discussions on the topic:

1. Bernie Dehler (atheist) and Christopher Townsend (Christian apologist)
https://youtu.be/1AqrqWIcmCI

2. Bernie Dehler (atheist) and Fazale Rana (Christian apologist)
https://youtu.be/1gEBc_MF3Bg

3. Bernie Dehler (atheist) and Dennis Marcellino (Christian apologist)
https://youtu.be/b4M057uBrwY