On a tan-pointed (atat) dog, masks can be detected by looking at the facial points. Tan normally occurs on the sides of the muzzle and above the eyebrows, but a dog with a mask may have all or part of these points covered up by the main coat colour. Usually, however, the tan on the neck is usually still visible. Masks can vary greatly, covering anything from just the end of the muzzle to the whole of the muzzle, eyebrows and ears. On sable dogs, black masks may also cause black hairs on the chest and/or the back and tail. Sables with black tipping often have black masks, so it is possible that masks in fact cause some of this tipping. However, there is probably another modifier that helps with this, because not all black-masked sables have extra black, and indeed, many breeds with masks come only in clear or almost clear sable (e.g. Pugs).
Resources on dog coat patterns and the genetics thereof.
Suggested by @metellastella
