[matmp]

Create a custom column
(find in : [URL=https://www.reddit.com/r/Calibre/comments/s48dzy/how_to_find_entries_that_dont_have_actual_files/[/URL])

Open "Preference" in Calibre, then "create custom column"
Choose "quick start" then "format"

Rename title like "Have Book" (as you want !)
Column type : let by default
Show checkmarks
Description : "Epub exist or miss"
Template model : {#formats:test(yes,no)}
Replace "Text" by "Yes/No"

Ok
Apply
Restart Calibre

You'll see in that new column if the book (epub) exist or miss !