There's no real practical way to distinguish a self-published KDP book from one which got to the catalogue via the wholesale channels.
The best you can really do is to see if the publisher (which people can just make up anything they like or use their old print-publisher name or whatever) is a "real" one with corresponding print titles and/or books distributed to other stores, but even that can be iffy given the amount of CreateSpace-type self-paperbacking out there. Generally I go by whether or not the "imprint" has more than a handful of authors in its stable and how broad a variety of titles they seem to have (otherwise I assume made up, and they're all buddies and/or glorified vanity press if their catalogue is really disparate and the books seem to be completely random and unrelated).
You can, however, distinguish the KDP Select titles which are self-published as exclusive to Amazon if you have a US-based account (or log out and pick the US as your country from the region dropdown on the Kindle store page) as in search (and only in search) on the resulting listings, you'll see "read free with Amazon Prime" or a similar phrase next to the KDP Select exclusive titles.
(This no longer shows for other countries, but we get the full-priced Prime lending "free" books displaying along the $0.00 ones when sorting by price.)
As for the OP question, I generally don't buy my e-books from Amazon at all, no matter how cheaply they may be priced.
However, I don't absolutely rule out buying a KDP book, but it would really have to be something compelling enough to compete with the published-only-in-Polish-translation later favourite series works of a dead man whom I've been toying with the notion of learning another language to read, which is the
circumstance I previously cited for under which I'd actually bother to buy a KDP Select book.
Otherwise, their format is not worth my money and I consider the exclusivity to be a purchasing turn-off.