This special quality allows the creature to detect approaching enemies, sniff out hidden foes and track by sense of smell. They can identify familiar odours just as humans do familiar sights.
The creature can detect opponents within 30 feet by sense of
smell. If the opponent is upwind, the range increases to 60 feet, if downwind,
it drop to 15 feet. Strong scents, such as smoke or rotting garbage, can be
detected at twice the ranges noted. Overpowering scents, such as skunk musk or
troglodyte stench can be detected at triple normal range.
When a creature detects a scent, the exact location is not revealed, only its presence somewhere within range. The creature can take a move action to note the direction of the scent. Whenever the creature comes within 5 feet of the source, the creature pinpoints the source's location.
A creature with the Track feat and scent ability can follow tracks by smell, making a Wisdom (or Survival) check to find or follow a track. The typical DC for a fresh trail is 10 (no matter what kind of surface holds the scent). This DC increases or decreases depending on how strong the quarry's odour is, the number of creatures, and the age of the trail. For each hour that the trail is cold, the DC increases by 2. The ability otherwise follows the rules for the Track feat. Creatures tracking by scent ignore the effects of surface conditions and poor visibility.
Based on the entry in Monster Manual 3.5 Page 314.