Postby Psy » June 16th, 2009 6:30 pm 
			You tell by usage.  Given the number of naze da?!'s I've heard, I suppose "most polite" isn't the most accurate term--  Maybe saying it's the stiffest word gives a more accurate impression.  Still, in formal situations it's the word you'll hear and should be using.  This is how it is taught, by professional teachers who are also native speakers.  I wouldn't go doubting them on a bet.
			
			High time to finish what I've started. || Anki vocabulary drive: 5,000/10k. Restart coming soon. || Dig my  
Road to Katakana tutorial on the App store.