Ofaction, a sense of smell, in frogs is mainly used as a homing tool or recognising breeding areas, but not often for detecting food.

Many frogs can sense chemical changes in the air, not just from the nostril but from molecules collected on their eyes and skin. Many tadpoles use picking up chemical 'scents' to detect predators and food. And learning the chemical smell of the pool they were born in, to return to later as frogs to breed.

A frogs nostrils are primarily for breathing through rather than smell as they breath with their mouths shut.

Frogs have a smell organ in the roof of their mouths called the jacobson's organ. This detects food. Sometimes frogs will open and close their mouths to try and locate food.

Taste

Frogs and Toads aren't very fussy eaters but will occasionally reject food that isn't right. They can sense the four basic tastes,bitter, sweet, sour and salty. This helps them reject unwanted matter they may have got in their mouths whilst feeding, or poisonous insects.