Did you ever have any doubt on the so-called reference letters of the domestic helpers or provided by the agent? My experience is that they will not let me to contact the ex-employers and they will give my phone number to the ex-employers to contact me. Any experience?
The agency may not given you the tel number directly, but you should ask the maid during in the interview, and try to make a phone call in front of her.
You should check the ex-employer info from the last contract and maid's passport. You will get the name and address of ex-employer.
Don't trust the agency, if you don't know them ver well. Some of them only make money, no service!