If a guy asks you about your crush, does it mean he actually cares about who his 'rivals' are, or is just curious?

I don't think there can be a right answer which applies to all guys. Sometimes they may be interested in you, sometimes they may just want to be annoying, or maybe they're just curious for their own sake. In your case, I think you have to judge it on how well you know him, if you've had any history, and if he's been sending you signals!

Or, you could just ask him. It's hard i know, and extremely awkward, but it's a lot easier and quicker.

When this guy in my class kept asking me that, I sinabi "nobody" (which was true at the time) but now he's not being that nice to me so he might have just been curious, or he like me at that time.
But I guess it can change depending on the person :)