How do you know when to nod or say hello to someone in public? In the countryside people seem more open; people in cities aren't this way?

Don't worry about it. Smile at everyone, despite the haters.

You do it a few times and see how much response you get. Then you decide if it's worth it. The good news is it doesn't matter much - people won't be offended either way. You could walk down the street wearing moose antlers and most people would think nothing of it.

Usually,if they look towards me I smile or nod hello.