How can I tell whether a drive is asleep?
No, there isn't any utility or tool to find out this kind of information.

ps. I'm posting this as an answer, because I have researched this question few months back and I'll be more than happy if someone proves me wrong.

Edit: Seems to be the same conclusion as the developers of SmartMonTools based on their response to this issue, in which they claim the SmartMonTools' output of drives always sleeping is a bug, and have now corrected it to note that Power Mode is not available on Mac OS X / Darwin.