Back when I worked in an office and conducted interviews, I made a point of always having the candidate sit in front of a computer and solve some very simple problems. It's amazing the number of candidates with apparently strong CVs who simply did not know how to use a computer. I mean, people claiming Linux experience who didn't know how to type commands at the prompt, not even "ls". How were they expecting to get a Linux developer role?? Unfortunately you have to weed these people out, and this was the only way I found.