In the US, certain things are ritual greetings that follow a standard script. Deviating from it is considered a bit weird (but it’s…
I think eye contact and posture plays into it. If they seem really focused, they want a more definitive answer. If not, it’s just a recognition that you are there.
I think I probably use different cues because I don’t generally look at faces very much. I wish I knew what mine were! I will have to pay more attention explicitly and see if I can figure it out.
If you don’t usually look at faces, I think there are still a variety of ways to know if someone really wants to have a conversation. They’re really different, depending on the situation. If someone stops what they’re doing or slows down to ask you what you’re doing, they probably really want to know. If their body is leaned toward yours, they probably really want to know. If their voice is low and flat when they say, “what’s up?“ they probably want a short answer. I’ve noticed that when people want a long answer, they say, “what’s up?” where the word ‘up’ sounds slightly higher. These might all be really obvious. And there’s others, but these are the ones I can think of now.