I like the following approach:

First ask them some basic questions to filter out the utterly clueless. E.g.:

• reverse a string
• replace all a's with b's in this string
• write a recursive factorial function and explain problems with it

If they pass that sanity check, give them a small test project that takes about an hour or two to complete. Let them use their own computer, tools, and the internet, and see how they solve it.