That info is based on the lightness or luminance of a pixel, depending on which effect is used (luminance is better, since it is perceptionally weighted). So if you switch to view that channel, e.g. by ways of the Channel Combiner effect, the whites, blacks and greys will give you some idea of which is which...
Thanks for the tip!
Thanks for your help Mylenium!
Another way to do it:
Apply Color Finesse to the clip in question, then enter the full Color Finesse UI. Click the Luma Ranges tab in the upper right.
Shadow areas will be rendered as black, midtones and gray, and highlights as white.
Not that there is such a sharp difference between the definitions of shadow/midtone/highlight, but it gives a nice quick idea of what's what.