1. Is it marketable?
To determine the marketability of your invention, conduct your own personal market research. Or, if you know an expert in the field, ask that person. The point is to figure out whether potential consumers will have any interest in the device. When you ask for opinions, avoid taking criticism personally, and try not to argue when an evaluator makes criticisms. For more on commercial potential, see Nolo's podcast Can You Patent Your Invention? Part One.
2. Did you invent it?
You can obtain a patent only if you are the inventor or co-inventor of the invention. A co-inventor is anyone who makes a contribution to at least one novel and nonobvious concept that makes the invention patentable. In patent drafting terms, the co-inventor must contribute something substantial to one of the patent claims.
3. Do you own it?
Just because you invented something doesn't mean that you own the invention. This is especially true if you invented something while employed. In general, your employer may own the invention if it was made during your term of employment, is related to the employer's existing or contemplated business, and was made on the employer's time.
4. Is it useful?
You can obtain a patent only if your invention is useful. Under our patent laws, artistic or aesthetic works -- though they may entertain, instruct, or amuse us -- are not considered useful. Assuming that your invention does something -- that is, it produces a result or makes a product -- you should have little difficulty establishing usefulness.
5. Does it fit in one of the patent "classes"?
Every patentable invention must fall within one of four classes defined by statute. The four classes are: processes and methods, machines, articles of manufacture, and compositions of matter. If you have a functional innovation, you probably won't have any problem demonstrating that your invention fits within one class.
6. Is it new?
Your invention must differ from all previous inventions or existing knowledge to qualify for a patent. If your invention differs physically in some way from previous inventions, it will likely be considered to be new (or "novel").
7. Is it something that is not obvious to other inventors?
All inventions must meet a requirement of nonobviousness. To figure out whether your invention meets this test, you have to consider whether people working in the field would consider the invention obvious.
For step-by-step help in filing a provisional patent application, use Nolo's Online Provisional Patent Applicationservice. It completes the required government forms, files your application, and then sends you the completed PPA with detailed instructions.