If a person looks just below the horizon (maybe 15 degrees below) and to their left (or to their right if they are left-handed) they are looking in an area that will stimulate their brain to access Auditory Digital information related to sounds they remember and stimulate internal talk. This is a sign the person you are talking to may be having a conversation with themselves. I happen to have a perfectly balanced brain (I'm ambidextrous.) so when I look directly straight like this, I could be having a conversation with myself. The key is that I'm looking down a 15% grade from the horizon. Then you want to figure out if I'm right handed or left handed to determine if I am having a conversation with myself.
Try this: What was the first thing you said out loud today? Most likely to answer this question will have you eyes position themselves just below the horizon and to your left (or to your right if you are left-handed).
It is thought that 15% of people in North America are dominantly auditory in the way that they learn and process information. This doesn't mean they aren't capable of using their other senses. They just have a preference. And, even though they dominantly use visual ways of processing it doesn't mean that they don't go through other senses to complete their information processing. Generally, we use many more of our senses to process information. So 15% of North Americans (maybe) mostly start auditory and then use some other sense and and then another sense and on and on - This accounts for our very different and unique learning and information processing styles and explains why so many people these days are diagnosed with learning disabilities who aren't really learning disabled.