When talking about traits, we are talking about a character's personality traits. Personality is the way in which a person behaves, thinks, and even feels on a regular basis. A person's personality is mostly shown in the things they do and say. So when analyzing Juliet's characterization for her traits, or personality traits, what we are doing is examining the things she says and does to see what they reveal about her nature.
One good scene to analyze is the very first scene in which we meet Juliet, Act 1, Scene 3. In this scene, we see her interacting with her mother and learn about her present views of marriage. When interacting with her mother, we see that she is very respectful and polite and also very forthright in her answers. For example, when summonsed, Juliet answers with, "Madam, I am here. / What is your will?" (7-8). Formally addressing her mother as "madam" shows us how polite and respectful she is, which are also some of her personality traits.
Also, when asked what she thinks of marriage, Juliet gives a very honest and forthright answer when she bluntly says, "It is an honour that I dream not of" (70). This one little frank response shows us that, while she respects her mother, she also values her own heart, mind, and self. She cares about her own opinions, so much so that she feels she has every right to make her opinions be known. Through this one little response, we learn one of her traits is self-respect. In addition, since it is such an honest, forthright answer, we also learn that other traits of hers are honesty and frankness.