The framing of this question is interesting. “…or are you just being nice?” Seems to assume that being nice is not a legitimate or authentic way of being, maybe unless it is a means of getting something you want.
What the OP is saying is "do you really care" or are you feigning interest because it is the socially acceptable thing to do? That's what "just being nice" means.
Fair enough. I find, for better or worse, there are very few people I care about to that extent.
When I visit with family this upcoming holiday season, I will do the kind thing and ask how things are going of my extended family. Not because I care, but because this is what is expected. My cousin is going to welcome his first child in the next few months. I am happy for him, and will offer my support in any way I can, but I don't care about what he's doing to prepare or "how he feels about being a dad."
Edit: I still think your scenario falls under "you really care, you are not being nice" in this hypothetical.