Essential Arms receivers have a J-15 Model Number, with raised lettering, except for the serial number, which was stamped. They are cast, but very good cast. I have never seen a malfunction attributable to the quality of the Essential Arms receiver.
They made ONLY receivers. A sister company, Essential Accessories, out of the same location, sold kits with and without lower receivers, but still not whole rifles. They never stocked barrels. Every other part, but no barrels. They would recommend several places to order barrels from, but I repeat, they did not stock or sell barrels... therefore, no complete rifles. All Essential receivers were made before the ban, since the company went out of business in 1993. Whether or not they were assembled into a complete firearm you can't say.
I had seen their kits and parts at gun shows, and they were top quality parts. But I have never seen an Essential Arms built with one. They would be easy to spot... their lowers, except for the last few years they were in business, were made in the old moly gray finish. It was offered in black as an option for a few years. Most were gray. They sold matching gray uppers in their kits and separately.
EVERY Essentail Arms rifle I have ever seen had mismatched receivers, gray lower, black upper.
The lowers sold back then for the princely sum of $59, or $55 if you bought in quantity. So, these receivers attracted bottom dollar builders who bought junk kits for $250 or so, made up from torn down, worn out M16's. (Those kit suppliers know who they are!) I have seen these rifles built with the trashy plastic trigger guards and plastic buffers, and internals that had been reparkerized.
Some of these lowers ended up under better quality kits, but ALL Essential Arms are kit guns. Some are true "Frankenguns".
Since it was not an issue, having to prove it was assembled before Sept of 94, before the ban, there was no reason for anyone to save receipts to "prove" it. I have not heard of any arrests or prosecutions on this one point.