How do you tell if sarcasm is mean-spirited or not?

I’ve always wondered…

Let’s assume for discussion that the sarcastic person is a person you’ve just met and that you have no other evidence to show whether the person was just joking/teasing or being mean. How would you tell?

e.g. They ask me a question. I give a diplomatic, non-committal answer like “Possibly” or “I’ll consider it” to which they say something like “Ya right” with a grin.

The reason why I give a more diplomatic answer is that I don’t know the person very well and I don’t want to risk saying something that would offend the other person.