While definitions of 'good developer' do vary, the ultimate proof is delivery of "good software" (possibly within time or budget constraints). Figuring that out without working with someone is hard. Most technology companies put a lot of work into that and still make mistakes from time to time, hiring someone who ends up not living up to their recruiting evaluation.

Even if you could accurately measure 'good developer', someone who meets that definition might not be available, and deciding if anyone who is available is 'good enough' can also be tricky.

That said, I think the foundations of the solution have already been posted here, so I'll summarize some of that and add to it:
- Accept that it's hard, and that you may make mistakes.
- Use any connections you have to get someone technical involved with the selection if you can.
- For this kind of work, also make sure they can explain software development and its challenges to you clearly -- you're going to need someone who can do that with you about your software.
- Choose an iterative process for software delivery so that you can see the progress as it happens and evaluate success.
- Make sure that if you're not happy, you can move on. Be prepared to do so.