I note, purely out of disinterested geeky curiosity, that this could be resolved (at least for a single title) if two people purchased the same title, removed the DRM, generated a hash of the titles, and compared just the hashes. MD5 would be adequate. No need for SHA-256.
But (and here's where reading too much John LeCarre gets you) there's always the chance that they only do it for some titles and not for others. And even if they don't watermark now, they could start adding it at any time! They might only add special tracking for people they feel are likely to be distributing books over the internets! It's a conspiracy wrapped in a cabal with a side order of racketeering!
Basically you can never be totally sure without checking each individual book. But you could reassure yourself that they're not doing it as a matter of course by doing the MD5 thing.
Don