Once you've found a prospective nanny, you'll want to cross your Ts and dot your Is.
The International Nanny Association has put together basic nanny standards: must be at least 18 years old, must have a high school diploma or equivalent and must be in good health (with proof of immunizations).
While those are a good place to start, you'll also want to conduct a thorough background check. Request and verify references from previous employers. Check work history, education records, driving record, criminal history (which will require fingerprints), credit report and Social Security number. Screening should also verify that your prospective nanny is eligible to work in the U.S.
This isn't a secret process -- keep in mind that your applicants can ask to see any screening you do.