2001: A Space Odyssey is the real litmus test
If you want to know who really is into not only space and astronomy but quite thoughful about humanity and the wider picture, either ask if they have seen the 1968 film 2001: A Space Odyssey (no offense to AC Clarke, but reading the novel does not count) or somehow get them to watch it.
Maybe it's a bit different today in the post Star Wars CGI world, but I have seen time and time again that those who like, love, or just plain "get" 2001 are far more into the above mentioned items than those who do not like, love, or "get" the film.
I know, it sounds obvious, but it will help you when we have to decide who gets to go into the space ark to escape the eventual desctruction of Earth in 2012.