Reading code is the biggest problem.

Good tests, I find, are to give them a simple piece of code with minimal comments. For example, some non-standard special purpose container implementation. Then ask them to explain what the purpose of the code could be as simply and short as possible.

Either - They'll start literally explaining each line of code without understanding the whole. - They completely miss the point. - They at least figure out it's a container of some sorts. - They'll explain technically what it literally does. - They'll give you the academic term for the concept it implements. - They'll actually explain the purpose briefly in simple words.

Those last three are good, in different ways. The better ones will also spot the bugs or performance issues in the code.