There is a couple of methods that you can use to help determine if a freelancer is the right choice for a project.
If you want to hire them for an hourly project, you can use the the tools that the freelance job sites have to monitor them. Most have webcam software to view the contractor and/or software that takes a picture of their screen so that you can monitor what they are working on. You can use this software to monitor the worker for the first couple of weeks to determine if they are working on your project and also see if they know their work.
Another method is to setup a small fixed price project to see if they can complete the work. If they can, then you can move on to more complicated projects.