Ask HN: As a student, how can I make sure a freelancing gig is legit?

I am not trying to be a freelance developer, per se, but I found an opportunity for some freelance Rails work that would fit my schedule pretty well (as a student). I have a phone interview on Wednesday this week.

I am weary because I see scammy looking job posts on my schools CS Facebook page pretty regularly (often business students who did all the work thinking of an idea and merely need a developer to implement it), and usually ignore them. But this one was upfront about the work and the pay, so I decided its worth a shot.

So my question is this: what should I expect the "hiring" process to be like as a contractor/freelancer? What signs should I look for to make sure this isn't a scam? Obviously don't work without a written contract, but what else?