[Bytecrawler]

I have well over five thousand ebook files (including duplicates) accumulated over 25+ years. I went though a DRM removal exercise about 15 years ago and decrypted most of the books I had then (keeping the originals), but over the years, files and folders have gotten mixed up and now I can't tell what is still encrypted and what isn't. Any of the old books that weren't decrypted are now useless - I don't have the keys or credentials any more - but I need a way to sort what is encrypted from what isn't.
I've tried loading all the files into Calibre and letting it tell me what is unreadable. In some cases, I can see right away since there is no metadata at all and the Title column just shows the file name. But for many of them, I do see metadata and the only way to know the file is still encrypted is to open it and let the reader throw an error. Does anyone have an idea how I can identify the encrypted files without opening every one of them (I suspect there is well over a thousand). The idea would be a plugin or something similar that would give me a new column with a DRM flag, but I'm open to just about any suggestion including using some other program to scan the files.