I created a sample file with a PictureField (container) and a calculation field, PictureFieldPath = PictureFile, result as text. When the container has an embedded image the calculation field shows the file name. When it is a reference it starts with Size = etc, and is followed by a lot of other info.
I would guess you could create a similar calculation field and search on it (for "size". It would be a tad unfortunate if one of your images was named 'Size...').
If you think there are only a few of one sort or the other you could export the contents of the PictureField and search for one type or the other, and re-find them manually. only
I use valuecount ( containerfield )
If it's an embedded file, count will be 1. If it's by reference, an inserted picture will return 3 and and inserted file will return 2. Inserted objects (windows only feature) will return 1 also, so you may want to include a different test if that's a possibile issue.