There are no ways to easily know if an ebook is DRM-free there, aside from buying/claiming it, and checking in your downloads page (replace the domain for the region-specific one you want) if it mentions "Download available in additional formats" or the equivalent in a given language.

Akin to other projects to list DRM-free contents in platforms that don't make the information easily accessible, creating a list on a text document (initially on Pastebin, may change later), and this post is to report those findings.

Also the DRM state may change, so books initially either DRM-ridden or DRM-free may be different on a future check, so beware of those too.

The list: https://pastebin.com/zHftQzyQ

Added:

• A Saga of Sisters and Secrets (Mendocino Cove Book 6) (English Edition), by Nellie Brooks: https://www.amazon.co.jp/gp/product/B0CJVLF7DN

Added:

• こころ, by 夏目 漱石: https://www.amazon.co.jp/gp/product/B009IXKPVY