Health care staffing is one of the most lucrative businesses within the staffing industry.  It is important to do your research and find a company that has a proven track record.  Making a mistake in health care staffing can literally cost lives and dollars in lawsuits.  Here are a few questions to ask prospective companies:
How long have they been in business.  5-10 years is ideal. 2 years is a prerequisite.

How many employees do they have. You need to watch out for the people that are working out of their house.  All they are doing is sifting through resumes on Monster and Linkedin accounts. 

How many clients do they have.
This will give you an indicator of how successful their staffing process is.

Do they have a case study.
This will let you read real results that were given to a past client.

Can they send you references.
If they can not send you references, then they have not done a good job in the past.