I found through experience and looking at many different inserts, that for the most part, Naturally faded inserts still retain their original luster and the fading is uniform and even.
Artificially faded inserts are for the most part, dull, lacking in luster and have splotchy or uneven fading (fading in the inner most or outermost)
I'm sure there are people out there who are really really good at fading inserts that can even trick the experts.
Naturally faded Fat Font Gen insert:
Artificially faded Fat Font Gen Insert: