Unfortunately no, it doesn't look like there are file-based differences between versions of ntpd that support leapfiles versus those that don't. Also, we can't forget that ntpd is shipped with OSes that do all sorts of wacky things with packaging, e.g. Mac OS X, so I don't think it'd be a reliable test.

I agree that this is stuff that Chef Sugar does, but I'm reluctant to introduce an extra gem dependency in this widely-used cookbook. My ideal scenario would be that Chef Sugar is eventually subsumed into Chef itself as first-class helpers, but that's a discussion for another time :-)