If the lining’s removable, take it out and look inside.
Sometimes the DOT notice is inside the helmet shell (although it should also have a sticker on the back I would think).
Re. companies spending time, money and resources making non-approved helmets, in some cases, that’s not what they set out to do. They design and manufacture a helmet (quite possibly for markets other than the U.S.A. primarily) but it fails the DOT testing when they import them into the U.S.A. They sell it anyway, to those not savvy enough to know to check (no offence intended).
Some helmets of course are joke helmets e.g. those little shiny black numbers favored by some cruiser riders, you know, the ones who have skull tattoos and wear “Bonanza” style leather vests. Those are not much better than wearing a do-rag, but they wear them as a kind of protest because the law says they have to. Those often *do* have DOT stickers, but it’s just a sticker, they’re often not actually DOT approved.