I would use Act II, scene ii. This is the famous balcony scene in which Romeo is being a peeping tom. He is watching her from afar in the first 40 or so lines. He describes her facial features and calls her quite bright.
He also refers to her great beauty in Act I, scene v when he first sees her at the party. He calls her the most beautiful woman he's ever seen.
So, if you feel like you aren't getting exact descriptions of features, do your best to infer what she must have looked like. Think about your own definition of beauty. Think of our shared definitions of beauty. She likely had a well-proportioned figure, fashion sense, flawless skin and similar features.
In Shakespeare's play, Romeo and Juliet, the reader does know that Juliet is nearly fourteen, so she still has an air of innocence and ingenuousness about her. After all, her father, Lord Capulet tells Paris to wait until she is fourteen to ask for her hand in marriage:
Let two more summers wither in their pride/Ere we may think her ripe to be a bride. (I,ii,10-11)
Being from Verona, Italy, she is probably of Norman descent and, therefore, is fair with light hair (blonde, perhaps) and blue or green eyes. (To this day, many northern Italians are of this coloring.)
As the previous post mentions, the balcony scene contains descriptions of Juliet that enable the reader to envision her. Her eyes must sparkle, for instance, for Romeo says that two stars twinkle in her eyes. Her cheeks are rosy and her skin glowing: "the brightness of her cheek would shame those stars" (II,ii,20). Even in death, Romeo finds Juliet "yet so fair" and "crimson in thy lips and in thy cheeks" (V,iii,95).
Of course, Juliet is reflected in the eyes of the amorous Romeo. Yet, she must be beautiful in a tender way, or else why would Romeo be so enraptured of her so soon after his heartache from Rosalind?
I'll add to an answer for you concerning Juliet's appearance in Shakespeare's Romeo and Juliet by just saying that there's really no way you can get much of an idea about what Juliet looks like from reading the play. Why? Because it's a play. Juliet looks like the actress who plays the part, whoever that may be.
She looks like Claire Danes in the version that was on cable last night, and she looks like Olivia Hussey in the 1968 (I believe) version. It's a play, rather than a novel, so descriptions are only given in extreme cases, such as a man with a hump or something.
She is young and Romeo considers her beautiful--that's about all you get. In fact, whatever figurative language Romeo uses to describe her is far more reflective of Romeo than it is of Juliet. Surely, she isn't really supposed to be brighter than a star or whatever.
As you're reading, then, Juliet's appearance is left to your imagination. As you're viewing, her appearance is determined by the actress playing the role.