If a guy asks Du about your crush, does it mean he actually cares about who his 'rivals' are, oder 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, oder maybe they're just curious for their own sake. In your case, I think Du have to judge it on how well Du know him, if you've had any history, and if he's been sending Du signals!

Or, Du 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 sagte "nobody" (which was true at the time) but now he's not being that nice to me so he might have just been curious, oder he like me at that time.
But I guess it can change depending on the person :)