In Odesk there is an explicit “interview” process where you ask the candidates questions. With other sites there are similar processes. Most likely when you get to this process you are pretty confident in the remaining candidate and have done the proper research.
The goal of this process is to discover the following:
Do they know and understand the importance of the project
Their knowledge and experience on the topic and style of writing
If their communication style matches yours
Either if you do not care to know those information, it is better to still do this process before hiring to make the candidate do more work. This will make the job seem more scarce and coveted and as such they will respect the job more and work harder.
Why do you think this project is important?
What kind of related work have you done before?
What’s your knowledge and experience with the topic?
Describe the target audience?
How do you typically communicate with your clients? How often?
How long do you think it would take to complete the task?
What would be your process for completing this task?
Do you have experience doing this work and how does it relate?
Do you have any suggestions for the project?
Do you have any questions for me?
Those should be enough to get their response and their dedication to the project. Then you can filter out the candidates and find out additional information to select the right one. This is great for hiring Virtual Assistants and other team members.