You simply ask the agent to ring the previous employer of the maid whom you intended to employ and then you talk with the previous employer of the maid.
Yes, the maid will have her previous employer contact number. If the maid was highly recommended, her master will let her to give their contact number to her future master to contact them directly.
**I had used a local maid and recommended by her previous master. I then referred my maid to another master with high recommendations and talked with her new employer.