How does Tybalt realize that Romeo is at Lord Capulet's party?
Romeo and Benvolio originally learn about Capulet's party when they come across the illiterate servingman in the street. He asks Romeo to help him with the list of guests who are invited to the party. On the list is the name of Rosaline, the girl that Romeo is in love with. Benvolio suggests that they go to the party so that he can show off other girls to Romeo in the hopes that his cousin will forget Rosaline. Before arriving at the party Romeo is worried as to what explanation they might give for their appearance at a party where they are not invited. Benvolio suggests that they simply go in and dance. They are also equipped with masks to cover their faces.
Tybalt recognizes Romeo by the sound of his voice: "This, by his voice, should be a Montague." Romeo has been talking to himself, describing the beauty of the girl he sees across the room who, of course, is Juliet. Tybalt immediately calls for his sword but is interrupted by Lord Capulet who warns him not to "make a mutiny among my guests." Capulet even describes Romeo in flattering terms saying,
He bears him like a portly gentleman,
And, to say truth, Verona brags of him
To be a virtuous and well-governed youth.