An illustration is like a fingerprint and is possible to recognise the illustrator from an illustration. Here is some examples.
Mabel Lucie Attwell-tiny children, pixies (signed).
Arthur Rackham: Dark colours, rather gothic in style.
Harry G Theaker: Pastel shades.
Aubrey Beardsley: Black and white, floral border
Kate Greenaway: bright colours, as if a toy theatre
William Heath-Robinson: Black and white, shows contraptions
Beatrix Potter: Natural colours, animals.
Maxfield Parrish: Like a stained glass window, shades that may not quite seem right.
Gwynnedd Hudson: Resembles old fashioned Christmas cards, 3D look, will have illustrations on the headline (the top of the page).
Margaret Tarrant: Similar to Kate Greenaway, but more defined illustrations.
E H Shephard: Black and white line drawings with fuzzy edges.
John Tenniel: Black and white images, notable shadows and patches of light.