Both of the following suggestions (by unboggling and by taustin) have merit but wallcraft's suggestion of using calibre's viewer gives you a nice error specifically saying the book is infected with DRM, see attached. Alternatively you can quickly check an entire library of ePubs using the
Quality Check plugin, see attached.
Quote:
Originally Posted by unboggling
Another way to know if they still have DRM is to try to convert them to a different format.
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Converting the file will give you an error and is a way to tell if the book is infected with DRM.
Quote:
Originally Posted by taustin
You can also rename (a copy of) the .epub part of the file name to .zip (because that's what it is), and double click it to open it up and browse the files inside.
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I'm sure unzipping it can tell you if it is infected with DRM, but renaming it to zip isn't needed there is a Tweak Epub (see attached) feature built into calibre that will let you explode the book and allow you to examine the contents.