Actually, nothing. It is not what Romeo says but how he says it which gives him away. She says, "My ears have yet not drunk a hundred words of thy tongue's uttering, yet I know the sound." She knows his voice.
"You kiss by the book." This was a good thing.
The friar tells Romeo to speak more plainly because he is not being clear about who he is talking about and which feelings he is experiencing. By speaking more plainly, Romeo can ensure that the friar fully understands the situation and can provide appropriate advice or guidance.
Romeo's servant Balthazar tells him the news from Verona.
Benvolio tells Romeo that Juliet is a Capulet at the party.
In Act 1 Romeo tells Benvolio that he is in love but his love is not reciprocating.
Benvolio(Romeo's Cousin), tells Romeo to forget Rosaline (Romeo's love before he met Juliet) and to go to the Capulet's party to meet other girls
Romeo's friend Balthasar tells him
Juliet's Nurse tells Romeo that Juliet is a Capulet, while Romeo's friend Benvolio reveals to Juliet that Romeo is a Montague.
Benvolio tells him.
at first she doesn't, but he tells her he is there eventually
Balthasar
The Nurse