Juliet obviously doesn't trust the parents who are forcing her into a loveless marriage. For a long time, she confided in her nurse. She sends messages between the two young lovers, covers up their secret trysts, and consoles Juliet on many occasions. Still, after Romeo's banishment, Nurse seems to switch sides and like Lord and Lady Capulet, encourage Juliet to marry Paris.
All signs point towards Friar Lawrence. He is to Romeo what Nurse is to Juliet. He offers advice, wisdom and (when he needs it most) a great escape plan. After her Nurse seems to turn on her, Juliet turns to Friar Lawrence as a last resort.
A day before her marriage to Paris, Juliet visits the Friar and pleads with him to help her get out of her marriage to Paris. It is at this time when the Friar suggests Juliet fake her death. Juliet had to have trusted the Friar or she would not have taken the poison.