folder.jpg exists indicator

For cover art, I generally have a folder.jpg in the album's folder and I don't include the cover tag in metadata. Is there a way from within mp3tag to see if folder.jpg exists?

There are 2 ways:

Disable "Don't display first image ..." in Tools\Options\Tags.
Then you can see the image in the panel.

or

Add *.jpg to the list of restricted files in Tools\Options\Tags.
Then you can see the JPGs in the columns.

I would revise this policy.
Embedding the covers has several advantages:
you can filter n MP3tag which files have a cover and which haven't
You can move the files whereever you want without them loosing the cover picture,
You are not limited to Microsoft players anymore (as they rely on this),
you do not get a resized folder.jpg file because the WMP has touched it and reduced it to 200*200 px,
You can create playlists and still see the cover.

So, what I would do:
load all files,
use an action (quick) of the type "Import cover from file" and enter
folder.jpg
as filename.
And then you can filter with
%_covers% MISSING
for files that have not got cover art - most likely the corresponding folder has no cover file as well.

Thank you poster (great username!) for the solutions and ohrenkino for your feedback. I started keeping cover images out of tags years ago to keep file sizes smaller in order to fit more music on my portable players. As player capacities increased, I kept doing the same thing just to stay consistent. However, I agree with the points you made.