How can you tell if a shrimp is in pain? A snake?

Animals are clearly important to us,so it's equally important that we avoid causing them unnecessary pain. For animals that are similar to us,
like mammals,it's often obvious when they're hurting.But there's a lot that isn't obvious,like whether pain relievers that work on us also help them.
And the more different an animal is from us,the harder it is to understand their experience.How do you tell whether a shrimp is in pain?A snake? A snail?