There are several differences between the Walsall and the De La Rue printings:
1. The background of the DLR is darker than the Walsall with the Royal Mail overprinting on the DLR more prominent.
2. The shading on the face, hair and neck and around the shoulders appears heavier on the Walsall printing.
3. The pearl in the earring is darker on the Walsall printing.
4. The background is printed as a a series of lines running top to bottom and angled to the left (a glass with 8x magnification is needed to see these):
On the DLR printing they appear broken and look like Morse Code, ie a series of dots and dashes;
on the Walsall printing they are more complete with only a few gaps.
5. The security cuts are basically the same except for the outer half-moon cuts. These are separated into two parts by a small tab (8x glass again needed). On the Walsall printing these tabs are 0.4mm wide, and on the DLR they are 0.3mm.
It is possible that differences 2 and 3 are exaggerated by the lighter shade found. If the shade varies between batches these other differences may not be as noticeable, in which case #4 and #5 may be more obvious.
I'd be interested in other observations as these spread round the country.
10 June: Ted has come back with an update based on labels from other sources:
The background shade is not constant and therefore should not be relied upon.
When the labels have been printed in the post office it makes identifying #2 and #3 virtually impossible, but there can be no mistake identifying #4 and #5 so the seven labels I hold plus the two I already had are all Walsall printed.