How do sharks know when something is food or not?
Studies have proved that shark senses are fully responsible for shark hunting.
They commonly use the electrosense for location, even at long distances.
The lateral line sense detects water vibrations usual to most living things.(see left)
The smell and hearing are also used for confirmation, leaving the sight sense the last check when a prey is going to be hunt.
After all these sensorial scanning, sharks sometime even test their food if it is not familiar for them.