I am not a textiles major, but:
1) Yes, ELS refers specifically to the fiber length, which must be, iirc, at least 1 3/4" long, on average.
2) Supima is a brand name, to promote the use of American ELS cotton. Apparently, this used to be called "American Egyptian" cotton, but that was before my time.
2b) Pima cotton is from the barbadense species of cotton, as opposed to upland cotton. It can be used as a marketing term, like pretty everything, from "shell cordovan", to "goodyear welted" to "vegetal dyed leather". Things have value because we assign them value. Marketing is merely an exercise to make a product relevant to a customer. It isn't necessarily dishonest, but it is not necessarily informative either.
3) You can't, certainly not without destroying the garment with tests on the material. And before that, you need to determine the metric(s) that you are using to determine quality.
Sorry the answers are not completely conclusive. At least the first and second answers contain some interesting facts, I suppose.
Wow. You're awesome. Definitely interesting facts. A few follow-up questions, if you (or anyone else) knows:
(1) Do longer fibres solely mean more durability, or does it also mean other desirable characteristics like, say, better dying susceptibility or more fineness?
(2) Who owns the "Supima" brand name? Is it owned by a conglomerate of textile interests or only, say, Brooks Brothers? Is it identical to Pima cotton or is it a subspecies of Pima? If so, how does branding Pima cotton as "Supima" do anything to promote American ELS cotton?
(2b) Does the fact that Pima is a "barbadense species of cotton, as opposed to upland cotton" mean it's better or worse than upland cotton and, if so, in what ways? My (limited) understanding is that expressions like "shell cordovan" mean something specific with respect to quality (e.g. it's derived from the horse butt, is tougher and more durable than cow or calf leather and displays a more pronounced patina over time - though I guess things get more granular when you consider tanning procedures and horse stock); "Goodyear welted" means (as far as I understand) that the upper is stitched to the sole using a specifically regulated and more thorough process than regular welting, so the upper stays fastened to the sole better and, hence, greater durability; I don't know what "vegetal dyed leather" means exactly. My point is that these expressions translate to something meaningful and specifically definable for the consumer. So, what about Pima?
(3) Ok. Makes sense. But, short of tearing up the garment or sending it to a lab, do you know of any quick and dirty tips to determine cotton quality? And, if I did tear it up and/or send it to a lab, do you know of any meaningful metrics to determine quality beyond staple length (i.e. fibre diameter, weaving or spinning process)?
Thanks LA Guy!