Observe their behavior. Do they experience frequent mood swings and outbursts, or do they tend to let most things that bother them slide? Notice how expressive their faces are, and whether they tend to be positive or negative in speech. Once you start to get good at it, try to predict how they will react in certain situations.
Ask them a thousand questions. Seriously. Take out a notebook and make a list of 1,000 crazy questions. Skip all the boring “what’s your favorite color?” questions and move on to “Do you believe in ghosts?” and “What person from history inspires you the most?” Because even when people know each other really well, they don’t know the answers to those questions.
Learn about their past. Find out how they grew up, how strict their parents were, and how they got over their first breakup. Learn their favorite cartoon as a child. Learn about their future. Learn about their goals and dreams. Learn what they would do if they won a million dollars. Learn how they plan to discipline their children (and if they plan to have any children). Learn where they want to retire to.
While knowing “everything” about someone is unappealing because there’s no mystery left, it’s really rewarding to know things about someone that people usually don’t tell other people.
Get creative about your questions, and have fun creating little games. One of my favorites is a game where I say, “Your word is…. pumpkin.” And then the person has to tell a story about something they did or that happened to them that involves pumpkins. With every turn, you change the word.
Never think you know everything about someone. Always challenge yourself to learn new things.