Generally speaking you want to focus on more than raw technical skill. It's far more important that you align on expectations, values, etc. than it is to hire the world's greatest developer.
Most importantly: pay them to work on a small project before tackling your MVP. You don't want to get involved with someone on a big project only to find out the person is difficult to work with.
Here are some tips:
1. Check for their LinkedIn profile and actually do attempt to talk with their clients. Do your research well, cold email should work.
2. See how old their clients are and if they are still in business. Substandard product reduces the probability of the project to succeed.
3. Involving external consultants to help you evaluate the Agency/Freelancer suited for your business. They would provide unbiased opinion which would help a lot.
4. Always go slow, it might even make sense to comission 5-10% of the project and see how they perform. It reduces your overall ridk.
5. Check their GitHub account for open source projects to gauge on technical competency.
6. If you can't afford an architect, I'd still recommend getting few hour worth of their time to sense how the project is progressing technically. They would assist like a consulting CTO.
There are scammers everywhere, the onus is on us to be diligent.
Shameless plug: I run an agency, email in my profile. Do send me an email if you need any help.
There are different kinds of people suited to these stages. First step, get someone to write out what you want to do, ideally a developer. Commission a scope of work document, or write one yourself, with things like what pages are on your application, what relation they take to each other, UI, and so on. It's better for someone in programming or design to do this. Get a rough estimate for each.
You'll probably have to pay a good amount to get that done, but it's not wasted time. It's wasted if they misunderstand and build something you don't want. It's wasted if you misunderstand how expensive a component in (Social media logins are an example of something more effort than people realize). It's wasted effort and money if you thought you wanted something, but then scrap it later because that wasn't a good idea after all. And defining the problem is a big part of development as well, one that gets glanced over even by professionals. You should do wireframes to define UI - basically little sketches for buttons and where they go to, etc, and not with fonts, color, and the like.
Once you get the definition down, you should have a good feel for how good the person you're hiring is. If they're dumb it will clearly show, and make you frustrated. If they're great, they'll add input of their own and improve your plans.
Then you get to work on the following stages - hire a designer, a tester, find a way to deploy it to the users, though some freelancers can probably do all of this solo. Once deployed and you get your feedback, you cycle back to the definition phase for the next features.
Be attentive to the long term cost when you hire someone else to build an MVP--there's often a lot of changes, updates and natural iteration especially for early stage companies--just make sure you set aside a budget for that if you anticipate iterating.
If you don't, you don't know enough to be a technical hirer/boss in this situation.
It's hard enough for someone who does it professionally for years to know why they should pick Flutter over Native, or Vue over React. Most people probably can't afford the best stack anyway. Let the freelancer decide what's best to build the product in.
