How to identify nozzle wear
I believe the little experiment made by E3D - the same link you provide - answers your question very well. Several points about wear can be found in this article. After printing only 250 grams of ColorFabb XT-CF20 (carbon fiber filament):


The nozzle diameter had increased markedly
The inner walls of the orifice (opening) showed deep sharp ridges and grooves
The tip of the nozzle had become critically rounded, and shortened


All of these symptoms were found repeatedly for standard brass nozzles.

In particular, I believe the last of these symptoms may be the one most easily identifiable without accurate measuring equipment (and without observing print quality).

With regards to reduction in print quality, these symptoms could be simulated by:


Setting the nozzle diameter too big in your slicer
Leveling your bed too high (the rounded tip will also reduce the length of the tip)
Printing with a partial clog that interruptus normal filament flow (due to the grooves and ridges)


Exactly what this will look like on your printed part is hard to predict, but I would assume you could see blobs, under-extrusion, poor layer adhesion, as well as an irregular surface finish of your top layers.