If the book is still on your Kindle and you have not opened it since then you can check the LPR (last position read) information which includes the last date it changed in one of the sidecar files associated with the book.
See
KRDS - A parser for Kindle reader data store files for information on how to locate and decode the proper file.
Just checking the last modified date of that file might also work depending on how it is updated by the Kindle. I am unsure of that.