Are they licensed and insured.
Have they successful completed engagements similar to yours, and can you speak with prior customers.
Are there prices within scale, and can you access their reputation from prior customers truthfully.

This may be hard to prove but you should report any suspicion to your state department that issues the license.

If the inspector missed things that they are responsible to inspect you may have some legal recourse.

I would also file a complaint with the Better Business Bureau.