Mephistophelean stench: charnel-house breeze, wolverine sphincter zephyr, mutant swamp gas all rolled into one, which very sadly comes from the mouth of one who does not practice optimum oral hygiene. Can remove paint, eyebrows and developing love affairs at twenty paces. Sleep, coffee, booze and kimchi-related variants are common.
A canary in a cage suspended from one's forehead is an accurate and reliable indicator for hellbreath. When the canary begins to retch and gag, brush your teeth, numbnut.