I've talked with a variety of people about this subject, so I'll tell you what they've told me.
Don't give them free support. If you do this, they'll continually want more and more. At some point, you'll have to stop it and they'll likely be mad at you for it.
Since you don't know how many hours you'd be supporting them, start at an hourly ...
Try to find a developer who values transparency and having a short feedback loop - so you can follow the progress closely.
This will enable you to correct misunderstandings early or terminate the project, if it's obvious that satisfactory delivery will not happen.
However, this will also require you being available to answer questions, preferably with ...
I don't think you can.
Much like hiring a contractor to remodel your kitchen.. there's no way to know if they will complete the job and do it satisfactorily.
You simply have to check references and work history and ultimately have a little faith. This is often why hiring experienced, and perhaps more expensive, workers may be better.