This, 100 million times. I used to be embarrassed to ask such basic questions, but so many people miss the most fundamental things that I don't apologize any more.
My favorite question to determine if someone likely has spent any time at all in C: ask them to describe how a C string is stored in memory. I find a lot of people who know some OO derivative of C will think they are, by definition, also C programmers, but this tends to utterly stump them.
Why is this important? Well, I'm a career OS engineer, so I'm biased. But if you don't have the faintest idea how a C string is stored, you probably also can't tell me how the stack works, or know intrinsically (without thinking about it) whether a given hunk of memory is on the stack or on the heap. I've done probably 1000 technical phone screens in the last 4-5 years, and I see a pretty clear pattern emerging: Java programmers (those whose primary instructional language during college was Java) are, by and large, awful programmers. Even if you put aside the fact that they don't understand memory basics, can't use pointers (and often argue with me as to whether that matters) and don't know how anything outside of Java works, they are just plain shite programmers with bad instincts and bad coding habits.
I have often found it necessary to restrain myself from telling a candidate that they should go back to their alma mater and ask for their money back. Seriously, college CS programs are doing an enormous disservice to their students by sticking primarily to Java.