Currently deciding whether to go with fantastic photos or color-tested horses "only" (as far as possible)

Face Markings

Masks, cobwebbing, and eye markings

Masks

Duns sometimes, but certainly not always, have LOWER dark face masks. 

Sometimes a grulla (dun on pure black) will have a completely dark head :

This lady has an upper AND lower, nice dark mask.  Karisma Diamonds of Intrigue, 2002 Dun Kiger Filly.  Sire:  Cherokee   Dam:  FireDance ,  Karisma Kigers   

If you see darkness (masking) on the UPPER face only, such as on the forehead, this is often "shading", and not a dun characteristic.

Click picture to see full size.  A grullo stallion, Mainly Merlin. http://www.grullablue.com/stallions/merlin.htm
from Todd and Toni Perdew
Cedar Ridge Quarter Horses and Beagles
www.grullablue.com

Cobwebbing photo from the website http://www.lonehawkappaloosas.com/dreamy2.htm 

She is Ee Aa Crcr  -- "Dunskin"

From a folder on this web called "colt_face"

from Todd and Toni Perdew
Cedar Ridge Quarter Horses and Beagles
www.grullablue.com

http://www.grullablue.com/foals/foals07/07alaskafoal.htm

Diane Pinney's Sulphur stallion, Sulphur's Khanketsuba.  Click the pic to see his striking cobwebbing!

EYE MARKINGS

Treating the eye as part of the face

A very fine example of eye markings on  HIGH COUNTRY FIREDANCE, courtesy of Karisma Kigers

Here is another variation of dun eye markings. There are several types!  This AQHA filly is Rick to a Tee, owned by Mary Haas of Haas QH's, on one of her first days wearing a saddle.

Mary should really keep her fillies out of her makeup!  This buckskin seems to have overdone her "eyebrows"... this is Cowboys Shining 209, also of Haas QH's.  Note: THIS IS NOT A DUN.  Not all eye markings indicate that the horse is dun.