When do Romeo and Juliet find out that they are members of the enemy families? Who tells them?

is the
first to find out that he has fallen in love with an enemy. After he has become deeply enamored
ofat the Capulet party, Juliet's nurse comes up to her at the same party and tells her that her
mother wants to talk to her.

As soon as Juliet is out of earshot, Romeo asks
the nurse who she is. The nurse says:

Marry,
bachelor,

Her mother is the lady of the house

Romeo is thunderstruck to realize Juliet is a Capulet and says
his life is now owed to his enemy:

Is she
a Capulet?

O dear account! My life is my foe's debt.

Juliet discovers Romeo's identity very soon after
this. She asks her nurse who he is. The Nurse goes off to find out and returns to tell her he is
Romeo from the hated Montague family. (It is interesting that, in both cases, the nurse delivers
the news,her role as go-between.)

When she finds out who her mysterious
beloved is, Juliet exclaims, as an aside:

My only love
sprung from my only hate!

Too early seen unknown, and known too
late!

The last sentence in theabove implies that, if
she had known who he was earlier, she would not have fallen in love. Now, however, it is too
late.