Check their CV. Do they have the proper qualifications? Do not take anyone's word that they have qualifications, even your attorney.
Find out if they are on the District's list of preferred providers - if they are, you likely don't want to use them.
Get references from other parents - you might be able to use a preferred provider, but be careful, only use them if they come reommended by other parents who have used their reports to get services for their chldren without going to court if possible.
Look at one of their reports - hopefully they have on with private information redacted or you may be able to see a friend's report.