Seeing is believing. Although it might seem premature to request examples of a web development company's previous work, you'll want at least one site they've completed before hiring them. If they haven't done any work in your industry, make sure you see a website built from scratch and not something off the shelf from Weebly or Wix.
Asking for their portfolio can help you gain insight into their process and perhaps even discover whether or not they have an established methodology for dealing with problems that inevitably come up on projects.