The easiest way to tell is from certain music players. Some will have spectrum analysis plugins. Another option is getting Audacity(a free program). You'll be able to see how low music goes with that program as well.
The tuning of boxes is highly dependent on various details of your specific install, so someone who tunes a specific enclosure in one setup, would tune a different box in another car, differently. I guess a general rule of thumb is that subwoofers can play below tuning, but not much lower. Subs won't get damaged solely from playing low notes, but that doesn't mean that they can't be damaged while playing low notes. For example, a subwoofer might exceed its xmax at rated power at 24Hz. That can possibly damage the subwoofer, but there's a possibility(maybe certainty?) that lower power will allow the sub to play that low without exceeding xmax.
I would say that most rap stays above 25Hz, but it really doesn't matter if you use your subsonic filter.