In the case of a phone screen, I honestly find it hard to believe that folks have so much difficulty believing that it's possible to get a good sense of a candidate's skill and knowledge without sitting over his or her shoulder and watching him or her code. A phone screen is a conversation. In most cases, a candidate who is faking it will have a very hard time describing in technical detail his or her work, and answering technical questions about his or her work.
Start with candidates who have good resumes. Ideally get code samples and/or a portfolio. Prepare for your phone screen so you can ask good questions. It really is that simple.