You are looking to hire a game developer to make your game idea a reality but don't know how to choose one.
Let me help you.
Most of the time, all games are develop using OpenGL. To speed up the development process, developers rely on Game Engines. Game engines take care of all the complicated parts of OpenGL. This allows the developer to focus on developing your gameplay sooner. There is no need to know how to add realism to a character, how to add graphics effects or how to animate a sprite. This is all taken care by the game engine.
Is it a requirement to know OpenGL in order to develop a game? The answer is NO. But having a good knowledge on Computer Graphics and OpenGL separates the good game developer from the wanna-be game developer.
Like any other software, game engines may contain bugs. What would happen if there is a bug in the engine that is stopping the progress of your game? Your game developer either goes into the engine and fixes the issue himself (assuming the engine is open source) or he waits until the game engine team fixes the bug.
You definitely want to hire the developer that knows OpenGL.
So your first question to your candidates is:
1)How well do you know OpenGL? DirectX? Computer Graphics?
Secondly, all developers have their own style of programming but they all must follow the same programming principles. The seven most important programming principles all developers should know are shown below:
Why is this important?
Because you may change developers along the way and developers that do not follow these programming principles usually end up writing spaghetti code.
Any new developer will want to keep away from working on your game if your previous developer didn't follow these principles. And if they do agree, you will have to pay a premium.
So your second question to your candidates is:
2) Do you follow Object-Oriented Programming principles in your development?
Finally, your third concern is about Documentation. This is very important. The Documentation is not for you nor for your players but for future developers that may work on your game.
If your game becomes a success, you may want to add more levels or features. If you decide to go with another developer, he will want/need code documentation. If such documentation does not exist, he will have to decipher the implementation of the previous developer.
If such documentation does not exist, your new developer will have to charge you more and will take longer to start development of the new game version.
So your third question should be:
3) Will you be supplying complete code documentation upon completion of the project?
Hope this helps you in your selection process.