On humans: I think in these modern times, most people have to learn by nurture to recognize danger. People ignore their instincts. But some individuals have retained natural instincts. they are able to pick up on chemical signals and body language, (although sub-consciously). How some people know someone is bad and it turns out they were correct even though other people think they were nice. What happens is you get a bad feeling but do not know why. The chemical involved is likely adrenaline.

On body language, When visual signs are not available, it would be something in the tone of voice that the sub-conscious picks up on. When someone has a bad intent and trying to hide it, the nervous system is giving them away. They sweat chemicals.

The classic fight or flight reaction. You could also talk about innate fears eg spiders.

I think instincts can be conditioned as much as they are inbred; this is the principle used in the majority of military training, in which the goal is to desensitize the person and condition him to become a trained killer who acts entirely on instinct.

examples of instinct: run from danger, eat after they are born, cry when they are unhappy or fly north in the spring(birds).

The fight or flight reaction is insigated by adrenaline which activates the sympathetic branch of the ans. This enables us to better respond to stressors eg increased heart rate, pupil diltation, increased blood to muscles etc