I'm with minimus, some Jehovah's Witnesses do come across as smug. I've had Witnesses sorta smirk at me in discussions when they thought they had the upper hand.
I've also been able to pick them out at a coffee shop or diner by their outfits, in Upstate New York Brothers always seem to be wearing grey pinstripes with colorful shirts and ties (don't know if that's true elsewhere). Then I'd listen in as I was having breakfast with my family and hear them talk about the latest Watchtower or someone going out west to "unassigned territory" and now I'd guessed correctly.
I've never figured out someone was a Jehovah's Witness by what I'm about to say but it's worth mentioning that in my experience no one but Jehovah's Witnesses talk about "Bible books" or use "Bible" as an adjective for that matter ("we learn Bible truths"), other Christians tend to say "books of the Bible" and "biblical truth."