Things to look for when choosing a design company:
1) Are they certified designers. Organizations such as CEDIA and Home Acoustics Alliance (HAA) certify members who meet qualifications. For CEDIA, you may want to make sure they are Home Theater Design Certified. There is a major difference between a CEDIA certified designer and a CEDIA certified HOME THEATER designer.
2) Have they taken industry courses such as HAA, THX, CEDIA and ISF and are they certified in those courses? Are they CURRENT in those certifications? Plus, what experience do they have in applying the knowledge they learned from such courses?
3) Interview them. Can't stress that enough. Designers who know their stuff and those who don't will likely be readily apparent. Anyone can say, "oh yeah, we do that design stuff".
4) What knowledge do they have outside the AV industry? Are they engineers, scientists,
IT, architects, interior designers, what?
5) What do others have to say about them? Can they provide references if asked? Can you view their work if asked and if possible?
6) Are they willing to work within your budget?
7) Get a feel for their perspective. If they start talking about equipment right off the bat, then you know where they're coming from. Most "designers" sell from the perspective of a box rather than the science.
Just a few random questions to get the creative juices flowing.