Summarize:

Are they confident in their habits? Do they display distinct qualities that most people do not possess? Do you know the person rather well? If so, this person may be a good candidate for analysis. On the other hand, does the person tend to act like everyone else? Is the person shy? Is the person quiet? Think about people you know very well and compare this person to them. Consider many personality traits, physical traits, and how they respond to stress. Compare this information to people that you know intimately to help you guess their sign. It is especially important to observe their behavior while they are in conflicts or while working in groups. We all learn to hide our inner feelings to some degree while in public. People usually display their astrological traits in relationships and tense interactions.  For example, Aries' first reaction when faced with conflict is to yell or threaten, Gemini will talk his/her way out of it, Cancer usually responds with an emotional outburst, Taurus responds to conflict with silence and a posture of strength, etc. Cardinal (Aries, Cancer, Libra, Capricorn): These people usually responds to conflict with a bold, decisive, instinctive action in an attempt to trump the assault. They are easily motivated to start tasks. Fixed (Taurus, Leo, Scorpio, Aquarius): These people tend to hold in their reaction and respond that the attempt to hurt them was futile, with passive aggressive behavior or with silence. They concentrate on finishing things well. Mutable (Gemini, Virgo, Sagittarius, Pisces): These people tend to be artful with conflicts, and are known to try to avoid arguments. They are good at motivating people. Aries is considered the youngest, while Pisces is the oldest. The younger signs tend to display quick tempers and be more selfish and eager to learn while the older signs are wiser, more compassionate and more set in their ways. This is where people get the term old soul. Base your prediction from your study of astrology. The average guesser should have a 1/12 guess rate, but any study of astrology, however small, will improve on the 1/12 rate. Keep studying astrology from experts and from your own observations and your "guessing" will be 1/11 - 1/10. This will help you verify your guesses and improve your skills. You can use Google to find a birth chart calculator.
Look for people with strong personalities. Compare them to people you know. Observe their behavior. Pay attention to the age of the person's sign. Practice guessing people's signs. Ask the person's birthday and birth time.