I'm not sure what your background is, so I'll walk you through the process as though you had no experience. When evaluating an Android developer (or development firm), knowing the process through which a successful app is developed will help you tremendously.
Firstly, understand that a single developer is always going to be riskier than hiring a firm. While hiring a firm is scary for most people (mainly because of the perceived high cost), most firms have the product development experience to make your project a success. Individual developers may have the technical knowledge, but that's only half of the story. Without a real process (ie: Information Architecture -> Systems Design -> Graphic Design, etc), your project is much more likely to fail or never get off the ground. And once you've given money to an individual developer, you're very unlikely to get it back.
You'll also find the likelihood of having your idea "stolen" is much higher with an individual developer than with a firm. To most firms (mine included), creating apps is an internal investment, and we're far too busy trying to keep the lights on than investing in others' ideas. We have 100's of our own that we'd love to do, but just don't have the time.
You can find reasonably priced boutique firms in most cities. Look for one that specializes in the kind of software you want to write. For example, if you're looking for Android development, hire an Android development firm. They're out there (Disclosure: I own one), and you'll often get a much better price than going through a larger general purpose development firm (ie: a web development firm that does mobile). Just make sure you vet them - see what previous projects they've done, and talk to them before you commit any money.
Once you decide on a firm (or person), have them walk you through their development process. If they don't have a process, that's a big red flag and you should walk away. The process should have some form of the following steps:
1) Requirements gathering / feature development
Description: This is where the developer gets an understanding of all of the facets of your idea. They need to know exactly what you expect. Some development firms use user stories as a shortened way of doing this, while others may take longer and have a more formal process. In my experience, both are valid.
Deliverables: A list of the features that your software should have. My firm will also develop some basic marketing materials for certain customers, such as end-user profiles and target market data. This is not required, but can be helpful.
2) Information Architecture:
Description: This is where the developer creates wireframes to show how the end user will use the software (or app). This can include workflow diagrams, use-case scenarios, and wireframes. The least it should contain is wireframes. This is important, as it provides a basis for understanding how end-users are going to interact with your app which will ultimately determine how useful and usable your app is.
Deliverables: A project proposal. The developer should be able to give you a solid estimate on the price of the rest of the project after this phase.
3) Graphic Design:
Description: This is where a graphic designer takes the wireframes and information architecture documents and turns it into an appealing visual product. Graphic designers will keep the fundamentals of usability in mind, and should have a good eye for color combinations. It's easy to get side-tracked here, as this can often determine the marketing of a product, so make sure you understand the target market for your application very well before this part of the project commences. You generally won't reach 15-24 year old heterosexual males if your app has a pink unicorn theme.
Deliverables: Design Comps. These are usually Photoshop, Illustrator, or sometimes PDF files. These are exact representations of what the final app will look like, so don't be afraid to make tweaks. Understand, however, that sometimes an exact look is not possible in the programming phase, so try to encourage a design that can be "2 pixels off" if possible.
4) Software Architecture, Design, Development, and Test
Description: Finally, the meat of application development. This is usually all that a single developer will keep in mind when giving you an estimate for software development. In fact, as you can see, this is a later stage in the successful Software Development Lifecycle process. This requires a substantial knowledge of software engineering best practices, along with technical knowledge of the specific platform on which the software is being developed. Again, vet anyone you have doing this work -- ask them specific questions about how they plan to implement the software, and have them walk you through it.
Deliverables: A finished product, Project Schedule, Burn-down charts, and more. There are plenty of artifacts that are generated by a good development process. What you want to know is the process. Did they test the software? On what platforms? Is that adequate for the size of your market and your budget? Do they perform integration? Where is the source code?
Some other things to consider
Make sure you have a good contract. There are plenty of software contract examples out there. Make sure you find one specifically for Software Consulting or Product Development. Software is a new area of law, and there is a lot of contention about software ownership that can get you into trouble down the line. Make sure you are the sole owner of the software they create (called a "COMMISSIONED WORK" or "WORK FOR HIRE"). If you have any questions, don't be afraid to run it by an attorney. They'll help you find the caveats. We use a standard contract form from a company called "ContractEdge", which produces excellent contracts that are protect both sides of the arrangement are appear to me to be very fair.
That's my $0.02 -- best of luck with your project. If you're in the Southern California Area and need the best Android Developers, feel free to contact me. Android is a complicated beast, and we've been there since the beginning.