With there tongue! They flick their tongue in and out of their mouth to sence the air when the tongue is out it picks up scents and when it is flicked back into the mouth it rubs against the jacobsons organ which converts the sences and allows the snake to understand its surroundings
It's the Jacobsons Organ
They don't ! They flick their tongue - which catches scent particles floating in the air. the tongue is withdrawn into the mouth, and the tips inserted into 'Jacobsons organ' - a sensitive area on the roof of the mouth.
tongue
Their tongue and the Jacobson's organ.
The Jacobson's Organ - is a sensitive 'pit' located in the roof of a reptiles mouth. When a reptile flicks out it's tongue, tiny scent particles stick to it - and are drawn back into the mouth. the tips of the tongue are placed into the Jacobson's Organ, and the reptile 'analyses' the scent. This organ is so sensitive - it can even distinguish if the scent on one side of the forked tongue is stronger than the other side !
hearing
scales or skin of it
The Nose.
Jacobson's organ
Heat pit
There isn't a specific 'organ' - Snakes usually have a row of heat-sensitive 'pits' (or tiny holes) arranged around the top edge of their mouth.