Dreams. I've seen dreams reveal so much about a person. Tell me your most recent nightmare, and i'll see if i can deconstruct what it means. The more details you give, the more accurate. Tell what you were thinking and concerned about while having the dream. Tell me nothing of what you felt when you woke up from the dream. I've seen people regret how they've treated family members who've passed, i see people who get warning signs in dreams of how they mistreat their significant others, i've seen all kinds of things. Regular dreams seem to be tips and messages, but nightmares generally seem to be warning of impending major screwups.
I believe when your brain isn't being dedicated to understanding the world around you, and when it has plenty of time to process things and create things, it just generally has a much, much higher IQ than your waking self. Sometimes we have trouble understanding what that second personality is telling us. It also doesn't seem to contain all (though it may have some) of the biases that our waking selves do.