Well mines borked and the way it was shown to me was you warm the engine up and rev the engine back and forth from below 1800 rpm to above which is the 'interface' between where the dephaser cuts in and out (i'm sure someone can explain that better). With the hood up if the dephaser is starting to go you hear a sort of wierd clattery noise (hard to describe) and as the oil gets up to temp it gets worse and sounds more and more like a tractor.
From reading a few threads on here it's not often they go completely, but if/when they do you're looking at a big bill, much the same as if your cam belt snapped