Although it's not directly described (apart from directly before the Fall in the Asurmen books), there are generally more hints dropped about pre-Fall Eldar than DAoT mankind so we should be able to piece together a reasonable amount through hints.
I've got a couple of references that i can think of, but i'd greatly appreciate anyone else weighing in as doubtless there's tons i've missed!
Apologies for the oncoming wall of text! Anyone who gets through it shall get a medal
First off, i think it's fairly clear that both CWE and DE eldar society is radically changed from what the Eldar were like pre-fall. In the Asurmen book, it shows that the Eldar Aspects and the Path system as a whole were created as a postfall coping mechanism. Secondly, the Dark Eldar are described as being closest to pre-Fall eldar of the two main factions (with Corsairs being described as closer still), but they're described as 'closest' rather than 'close' and so much has changed in Dark Eldar society post-Fall that i'd be willing to bet it's mostly unrecognisable (replacement of the noble houses with kabals etc.).
To map things, i'll fairly arbitrarily split pre-Fall history into early (during the War in Heaven), middle (after the War in Heaven but before the slip into hedonism) and late (during the hedonistic period).Immediately pre-Fall and the Haemonculi
We know that during late pre-Fall eldar society there was rampant violence, pleasure cults and various other nasties running alongside more traditional aspects of the eldar culture.
From the old Dark Eldar codex we know that there were individuals known as Dark Muses
, which subverted power in late Eldar society. Commorragh was also formed in the early days of this period as a refuge from the law for the earliest pleasure seekers. We're also told that the Haemonculi have their origins in the masters of pre-Fall eldar society. Are they the descendents of these Dark Muses?Pre-Fall Eldar warfare and the nature of Khaine
We know that the Eldar themselves didn't really partake in warfare during the late period, their battles being fought by legions of psychomatons
which i picture as sort of close to wraithguard, but powered by summoned daemons derived from their gods (probably the war god, Khaine). What if the eldar myth of Khaine declaring war on the Eldar is a racial memory of an uprising of these psychomatons?
In their early period during the War in Heaven, the eldar are described as fighting the Yngir (Necrons) alongside Khaine himself. Could that actually be mortal Eldar fighting alongside these psychomatons infused with the daemons of Khaine? Or, if the concept of Khaine originated as an Old One (or more likely an Old One race/faction), were they fighting alongside the warriors of the Old Ones in this battle?
Also, my vision of the very early War in Heaven eldar warhosts is based on the concept of the Old Ones 'weaponising the warp'. In my mind, eldar hosts were primarily very powerful psykers summoning vast numbers of daemonic beings to fight, alongside powerful roiling psychic storms and bokts of energy. As their technology progressed, they developed shells of psychoactive materials (wraithbone) to bind these summoned beings into to create the psychomatons.
The description of eldar characters fighting the Yngir, the Hresh-Selain and the Autochtinii (all described as ancient enemies of the Eldar) indicates that early Eldar warfare was actually conducted by the Eldar themselves, possibly meaning that the reliance on psychomatons for warfare was probably a later development.Eldar vehicles, both civilian and military
We also know that they likely travelled around in skimmers. Dark Eldar Raiders are described as being derived from/reminiscent of the old pleasure yachts that Eldar used to cruise around in. It's also not a great leap to assume that Eldar Jetbikes were once civilian craft. If the eldar didn't really go to war themselves, then i doubt they'd have eldar-piloted armachines in ready supply. This probably means that all eldar vehicles are likely either converted civilian craft, or post-Fall inventions.Eldar societal structure
Also, Saim-Hann's culture is described as being similar to pre-Fall eldar society, meaning that they were likely a clan-based society with strong family ties. Also, we're told of noble houses that ruled Commorragh before Vect came to power, so as a society the had a royal elite.
We also know that Vect was born a slave in Commorragh before the Fall, indicating that slavery was present, at least in the dark corners of the webway, beyond the jurisdiction of the eldar high council (another snippet into eldar society).
We know that there was a significant religious aspect to pre-Fall eldar society, with priests and temples to their gods being present.
Also, it may be that at some point in their history the eldar were divided into two factions. In myth and legend, two warring families or individuals or gods are often proxies for historical warfare between two ancient factions (see theories of greek myths as indicative of cultural and military invasion). The houses of Eldanesh and Ulthanash could be two such factions, which according to myth were at war with one another before uniting against common enemies on two separate occasions (the Hresh-Selain first, and Khaine second).
Lastly, we know that the craftworlds were once massive trading vessels that plied the breadth of the eldar empire and maintained a differing culture due to their time away from the core worlds of the empire (allowing them to see the extent to which the eldar had changed). These craftworlds picked up survivors/refugees bith before and after the Fall, so current craftworld cultures are likely fusions of the original craftworld's cultures and refugee populations from different eldar worlds.Extent of the pleasure cults and the origins of the Dark Eldar
In the Asurmen books, i also gather that immediately prior to thee Fall, the core Eldar worlds were far more in the grips of the pleasure cults than the outer worlds. Furthermore, i gather that there are the first hints of what would become Dark Eldar raiding parties during the Fall indicating that either the practice of piracy originated then, or pre-dated the Fall.Technology
One of these proto-dark eldar is described as having wings grafted to him like Scourges so Eldar body modification likely pre-dates the Fall.
An eldar ranger interrogated by the Inperium said that 'the stars once lived and died at our command', indicating the possible ability to create and destroy stars with ease, although given the source that may well be lies and deceit to try and intimidate the interrogator.
Someone also pointed out that the process of reincarnation that the eldar could do pre-fall could be technologically based rather than an innate biological ability. The technology the Dark Eldar have now for resurrecting slain eldar is close enough to reincarnation that it could realistically be based on the ancient tech used for reincarnation, with cloning a later derivative.
We can also safely assume that the vast majority of the eldar technology tree was psychically based. All craftworld technology is psychically based, amd it's only due to the psychic atrophy of the Dark Eldar that they are forced to expand into other technologies. This would also explain why the eldar don't use their most potent weapons, legions of psychomatons and other pre-fall superweapons: they can't without attracting the attention of She-Who-Thirsts.
Any other mentions of what society was like? I haven't read the Asurmen books myself so likely there's more detail there. Does anyone have any more detail on Corsairs as they are described as being similar to pre-fall eldar. Speculation as well as existing fluff is also welcome