This one is a bit of a poser, every mash brewer tells me they can taste it, until it comes down to demonstrating this ability when they are judging beer in a blind tasting (like a comp).
Extract beers still pick up around half the places in any comp.
Personally I think there is no such thing as a uniquely identifiable extract twang. I want to qualify that by saying that stale extract, stale (slack) malt and antique hops all give bad outcomes; this is exasperated by cheep yeast and bad brewing practice.
My observation is that as progresses toward all grain brewing, a lot of things change, we use better ingredients, we control the temperature of the ferment our technique improves, we become better brewers, and we make better beer.
We do and should also become more critical of the final product; we also get better at identifying faults. I am not saying that a lot of bad K&K doesnt get made, just that it doesnt have to be bad just because its based on extract.
A fresh quality kit, good hop and yeast choices combined with good practice will make great beer.
MHB