The purpose of an aside is to give the character a chance to say something that either should not be heard by other characters or that would otherwise be an unspoken thought. Asides are generally very brief, usually no more than a few words or just a brief passage. Most importantly, asides are usually marked in the script by stage directions. Romeo speaks a very brief aside in Act II, Scene II when he says, "Aside. Shall I hear more, or shall I speak at this?" (II.ii.39). The reason why he says this is because he is thinking out loud to himself about what his course of actions should be next. The audience also needed to hear that he was thinking of responding to what Juliet had been privately saying; it gives the audience a chance to interact more with his thoughts.
In addition, the aside follows a much longer soliloquy Romeo delivers expressing to himself his private thoughts on Juliet. In contrast to an aside, a soliloquy is much longer and delivered at a point when the character thinks he/she is the only one present, while with asides, the character speaking knows that there are other characters present. Another thing that differentiates a soliloquy from an aside is that soliloquies reveal the speaking character's innermost thoughts, feelings, and motives. In some ways, Romeo's opening speech seems a bit like a cross between a soliloquy and an aside. One reason is because both the audience and he know that Juliet is present. However, regardless of her known presence, he has not yet interacted with her; she has no knowledge that he's there. Therefore, this speech is given by Romeo as if he is alone not really as if Juliet cannot hear him. It is also spoken more to himself rather than to the audience. These clues plus the fact that it is very long and that there is no stage direction indicating it is an aside make it very clear that we can conclude Romeo's opening speech is a soliloquy and not an aside. It is also in this soliloquy that Romeo first broaches the subject of responding to Juliet and letting it be known that he is there in the garden, as we see when he says:
She speaks, yet she says nothing. What of that?
Her eye discourses; I will answer it.
I am too bold; 'tis not to me she speaks. (12-14)
Therefore, Romeo's aside stating that he will now speak follows his first declaration of speaking to her, tying together well his thoughts with his actions.