After I stated that the sign is not as important as popular astrological culture wants people to believe, I want to give something back to the so-called sign, which is actually the position of the Sun at the time of birth.
Even though the Sun is only one among many astrological elements in an individual chart, being at the center of our system and being the giver of light and life, it's definitely important. Having a certain degree of astrological experience sometimes makes it possible for one to guess the sign of a person.
Many of us had the experience to spot a witty Geminis, a mysterious Scorpio, an energetic Aries or a sensitive Pisces. If the Sun sign is just one among several astrological factors, why is it easier to guess it but not as easy to guess Mercury's, Mars', Moon's or Saturn's positions in the signs?
The reason is that the Sun is more connected to the visible personality, as the Sun itself is the most noticeable element in the sky. Also the ascendant can be spotted more easily than other astrological elements, being connected to the body appearance and in some measure to the external behavior.
Since the Sun is also related to the ego part of the personality (“look how bright I am!”), the ego is what is often most visible in a person, even though many of us try to conceal it. We might also consider that the sign of the Sun is more easily observed when the Sun is one of the most important planets in a person’s chart. In case the Sun has only few or minor aspects, his characteristics would stay largely in the background.
However, guessing the sign of a person doesn't mean we understand their deeper traits. The sign of the Moon is a bit more difficult to spot inasmuch it's connected to sensitivity, imagination, feelings, and inner needs. Reaching those parts of the psyche requires a closer understanding of a person than just noticing the Sun personality.
Sometimes the sign occupied by Venus can be identified in the way a woman expresses her sensuality, by her charm (or the lack thereof), and by her modalities of expressing her femininity. Similarly, Mars in a man might be recognized in the way a man acts in his competitive endeavors, or in the way he manages power or conflicts.
Other planets are even more difficult to spot, especially the slower planets, which are less individual and more generational in their influences.
All in all, we always should remember that the sign is just one of the elements of a natal chart and that an astrological reading should consider the wholeness of planets and house positions and their relationships through the aspects.