I would point them to the ADA for a response.
Here is a link to a PDF produced by the ADA
http://www.ada.gov/svcabrpt.pdf
They cannot require any type of ID or certification but they can ask you what behaviors your dog has been trained to do.
Since she is a young dog and cannot do all of the behaviors she eventually will - you need to be able to clearly define what she currently does. You may also explain what behaviors you expect her to perform when she is mature.
I would provide the above listed PDF to the landlords and a description of her behaviors when you inquire about housing.
Sue