As with any online business it is a good idea to check out carefully ANY
SEO firm you intend to do business with.
My favorite check is to view their rankings for the keywords in their homepage title.
If they can't get rankings for themselves after being in business for sometime, they are suspect IMO.
And don't fall for that old line:
"We are so busy getting rankings for our customers, that we don'thave time to work on our site".
If they are so busy, how much time do you think they will spare for you?