How to tell the mood of a snake?

I know snakes might not have the same feelings as us humans or even as other mammals. They do have certain behaviors you can predict what mood they are in.

I'd like to know when a snake feel threatened, what do they usually do? How do you tell if the snake is being defensive, or aggressive, or scared, or just trying to relax and feeling happy?

What would you do in each of the situations?