What to ask an agency or a freelancer before you sign
You're about to hire someone to build something important and you can't judge if they're any good. These questions separate the ones who know from the ones selling smoke.
Hiring someone to build software is hardest exactly when you need it most, because if you could judge the work you'd probably do it yourself. You're going in a bit blind. These are the questions I'd ask if I were on the other side of the table, knowing what I know.
First: 'what happens if this goes wrong or you have to walk away?'. The answer matters less than the reaction. Someone who knows their job has thought about that case and talks to you about code that stays with you and how another team would pick it up if needed. Someone selling smoke gets uncomfortable with the question.
Second: 'who owns the code and where does it live?'. You'd be surprised how many people find out too late that their software sits in an account they don't control, or that they have no idea where the keys are. Before signing, put it in writing that everything, code and access, is in your name from day one.
Third: 'show me something you built that's been running for a while'. Anyone can put together a pretty prototype over a weekend. Keeping something alive for two years, with real users on it, is a different league. Ask to see that, and if you can, talk to that client.
There's one sign worth more than any answer. People who genuinely know their stuff say no to things. They warn you that what you're asking costs more than you think, or that for your case it's better to buy something off the shelf than build it. Someone who says yes to everything either hasn't understood the problem or doesn't plan to be around when the bill arrives.
None of these questions is technical, and that's the point. You don't need to know how to code to notice who respects you enough to tell you the uncomfortable truth before they charge you.