Building a piece of software (that's unique or not) is one thing. Running a business is a completely different thing. Do not worry about your idea being stolen.

That said, I would always suggest looking at a developer's resume and sample code (GitHub, etc.). They should be active. If you find someone without any online presence, they don't follow news or communities for things...That's a red flag.

For a senior developer, ensure they have 6+ years real-world experience. Ensure they have been exposed to a variety of tools and languages. Ensure they are enthusiastic about what they do and that they are a good problem solver.

Get a developer like that...And you're set.

Also try to find those who have experience with the same kind of tools you'll be using. A developer should be able to tell you the appropriate tools for your project. They shouldn't try to shoehorn it into something that doesn't make sense simply because they don't have experience with those tools. So you may need to have a general sense for your architecture first before hiring a developer. You want a very senior person helping you out with this (pssst, Clarity is a good place to for this...shameless self-promo - come ask me, I'll let ya know what you need).

Programming is NOT about sticking to one language and knowing every little tiny detail. It's not about committing some API to memory. It's about problem solving and organization.

When hiring, I rarely ask programmers to do silly tests or write out code on a whiteboard, etc. That's a meaningless academic way of assessing how well someone has memorized something (that likely does not even relate to your own product!). It has absolutely zero bearing on real-world web application programming. Plus, you won't be able to do that because you wouldn't know if it was good code or not.

So instead, I focus on problem solving. You can too. I throw out challenges and ask how would you do this? What kind of database would you use. Or which language? Would you need multiple languages even? What is the work flow? How would you deploy and make changes to the app? How would you handle a situation where millions of users were online and the app was crashing?

You can learn a lot by having high level conversations like that. Don't forget cultural fit and communication skills also play a huge role here.