Warning Sign Books The first thing I do when I enter someone's apartment for the first time--usually while the host prepares me my first drink--is take a look at their bookshelves. You can tell a lot about someone by the contents of their bookshelves, and the condition of the books therein. First, if they don't even have bookshelves, you've got your first major warning sign. This can be okay--many avid readers promptly give their books away once they are done with them, which makes judging them harder, but means that they do God's work, nonetheless. Check for piles of books sitting around, preferably open face down, or with bookmarks stuffed in them.
I find it very hard to trust anyone who doesn't have overflowing bookshelves, or stacks of books placed haphazardly around their apartment. It's probably a sign that, no matter what the size of their alcohol collection, we will ultimately not get along as well as we should.
Even those with overflowing bookshelves don't get away unscathed, though. You can tell more about someone by the books they read than anything else, I believe. And while I'm generally very forgiving, there are some things that are instant warning flags in any bookshelf, that should give any right-thinking person pause.
Atlas Shrugged, The Fountainhead, or, dear God, The Virtue of Selfishness. Any objectivist literature should be considered with extreme suspicion. Also, see if the execrable sex scene in The Fountainhead has been well-thumbed through: "The act of a master taking shameful, contemptuous possession of her was the kind of rapture she had wanted." If this has been highlighted, flee immediately.
The Bell Jar (women only). Not something to run away from, per se, but something to keep you on your toes.
Anything by Bill O'Reilly, Rush Limbaugh, Michael Savage, Ann Coulter, etc.
Tuesdays with Morrie and/or The Five People You Meet in Heaven
The DaVinci Code
These all only qualify as circumstantial evidence, of course. It's entirely possible and likely that perfectly sane people will have a wide-ranging variety of books on their shelf without it believing in all of it. I have multiple Bibles, a Koran, the Vedas, Catcher in the Rye, Das Capital, and Harry Potter on my shelf without subscribing to the philosophies contained inside these texts. But any of these texts--or all of them--should at least raise your eyebrows. You should check to see if the books have been well-read, or if there are any post-it notes, highlighting, notes in the margins, or other evidence that they've read repeatedly and subscribe to the beliefs of the offending material.
And then pay very very close attention to what your new "friend" says.