The only way you could know for certain would be if the images were linked
to their sources. Otherwise, what the images were originally and how Word
treats them once embedded aren't necessarily the same.
If you're using Word 2007, you can save the document in .docx format, change
the extension to .zip, open the .zip file in Windows Explorer, and drill
down to the word\media folder, and you'll see a list of the images contained
in the file. Files that originally were .jpg will be .jpeg, but files that
originally were .bmp will now be .png (from what I can see here).
