<p><a href="https://e3d-online.com/blogs/news/are-abrasives-killing-your-nozzle" rel="nofollow noreferrer">E3D-Online</a> and <a href="http://makezine.com/2015/09/11/carbon-fiber-filament-ruins-nozzles/" rel="nofollow noreferrer">Make Magazine</a> have written about the potential damage printing carbon fiber and glow in the dark filaments can do to your printer's nozzle.</p>
<p>What I can't seem to find is what clues or warning signs to be on the look out for if your nozzle has taken a significant amount of wear. I've printed a few hundred grams of glow filament personally and have not noticed any change in print quality.</p>
<p>E3D says you may have &quot;unpredictable erratic printing&quot; with a worn nozzle. Can anyone explain or provide examples of what this actually means and when a replacement is necessary?</p>

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.