Ask the candidate if he has contribute to an open source project. If the candidate has contributed (preferably non-trivially), it means that his code has been

Ask the candidate if he has contribute to an open source project. If the candidate has contributed (preferably non-trivially), it means that his code has been vetted by some very experienced software engineers and found acceptable. As well as the code vetting-by-proxy, you should be able to judge his "passion" (the current evaluation buzzword) by discussing what the projects were and what he did on them. You don't need to know what he is talking about in detail, although that would be good, in order to get a feel for his passion level for programming. If the candidate has not contributed to an open source project, this test is inconclusive.