I mean, this is pretty much the easiest way of handling the problem.

When I was teaching in a university the easiest thing to test someone's understanding was a simple question: "Explain me how would you use this in a day-to-day situation."

If the guy knows his business, he'll go on a long lecture obsessing over every new feature of C# 2.0 or PHP 3. He would give you all the differences between Firefox 3.5.1 and IE 6 with Maxthon. (Goddamn I'm freaking old)

Should you not find a spark of interest but just random mumbo-jumbo of words that don't constitute sentences, you can clearly see that there is no understanding present in the subject.

In this case you work as a teacher, helping the guy to figure out what he does not understand in a subject and simplifying things for him till this clicks. After that he will be able to talk about said subject for hours with gusto.