The first thing you should do is to ask to see their portfolio, that is, examples of other sites they've designed (ideally, they should be sites with a similar goal to yours).
A couple of other things that come to mind:
- I would want to be clear about ownership of the rights to the material they develop. (I would accept that the developers retain copyright in their creative work, but I would insist that my company has full rights to exploit that work in any way we please that is not detrimental to the developers.)
- I would want to be assured that we would have full access to the site (login credential, etc) so that we can maintain it ourselves (or hire another firm to do so) after the original developers have departed.
No doubt other folk here will come up with some more answers.