These are ubiquitous technologies. You are going to get flooded in enough waste that 20 mere questions, no matter how clever, are not going to help you too much.
First, find a developer whose sole job is to help you hire other developers, preferably a friend or someone you trust. They don't even have to necessarily be experts in, say, MYSQL or anything else you have listed there. Great embedded C dev? Great! Java master? Fantastic. Great news is that this person is not going to be doing any developing for you whatsoever so you can take on someone who has a day job without worrying about IP issues or anything...
Now, you are not going to want to waste their time (and your money...you should definitely pay your consultant and don't rely on favors) so step two is to put out requirements for some piece of functionality you need done. Ask for portfolios and resumes (but take them with a big grain of salt) and pick some with your consultant that seem to fit the bill. Pay the cash and let them get cracking. DON"T try to get them to do any work or take a test for free to prove themselves. That is a sure way to skim the cream off the top and get left with the dregs. If you can't come up with requirements, you've got bigger problems. By the way, this is a good place to shake your acquaintance tree...let it be known that you are looking for people to pick up small projects that could lead to a more lasting relationship.
Review and test their work. Pick out a few with the help of your consultant that seem to write good, functional code on time and with good communication. It is possible to have some major stand-outs here. If so, you might want to jump on them but try not to get too excited.
Now that you have your finalists, give them a larger task...perhaps a tougher technical problem if your needs drift in that direction or, more likely, requirements that are not fully realized. Have them email each other copying you on everything, as part of a team...pay attention to how they communicate with each other and hash things out. Who's having the ideas? Who is happy to coast and let others drive? Who's a bully? etc... THIS IS KEY! This is going to separate the merely good developers from the real value creators. If you end up with a bunch of people constantly begging for direction or flip flopping on every issue on how to proceed, you need to cast a wider net and start from the beginning armed with that much more experience. It might seem expensive but not half as expensive as anchoring yourself to the mediocre. That being said, keep the solid followers in mind when you need to fill gaps with freelancers. Ditch the bad attitudes and low performers and keep your value generators -- the key performers -- the stand outs that work well, create solid deliverables that make you very happy and work well together.
Treat your people well, rinse and repeat as you grow (shaking your team's talent tree as you do).
Good Luck!