Romeo climbed over the wall surrounding the Capulet garden to gain entry and see Juliet.
The only person who knows he is in the Capulet garden apart from himself is Juliet.
Benvolio
They don't, so it doesn't matter.
The line "Is she a Capulet?" is said by Romeo in William Shakespeare's play "Romeo and Juliet". He utters this line when he first sees Juliet at the Capulet's party and realizes she is from the rival Capulet family.
He goes to Juliet's house and breaks into her garden.
Capulet (Juliet) and Montague (Romeo)Montagues and Capulets
Capulet
Juliet, Romeo, Tybalt, Nurse, Benvolio Or . . . Juliet, Romeo, Friar Lawrence, Nurse, Capulet Or . . . Juliet, Romeo, Friar Lawrence, Capulet, Tybalt Or . . . Juliet, Romeo, Capulet, Mercutio, Tybalt Or basically Romeo and Juliet and any three of Nurse, Friar, Capulet, Mercutio, or Tybalt.
Benvolio tells Romeo that Juliet is a Capulet at the party.
Juliet recognizes Romeo's voice at the Capulet's feast.
The Capulet's illiterate servant asks Romeo, not knowing who he is, to read it to him.
As you suspected when you added this question to the category "Romeo and Juliet", the Shakespearean play which features the Capulet and Montague families is in fact Romeo and Juliet.