How do I get the DPI and resolution of device?

I use capabilities.screenDPI in every app, and have done for some while.  It seems to work ok!

Specifically, I use this snippet of code to detect if I'm running on a screen larger than 6", and then I use that to scale down my UI so that it doesn't look enormous on larger screens:

var screenWidth : Number = Math.max( stage.fullScreenWidth, stage.fullScreenHeight );
var screenHeight : Number = Math.min( stage.fullScreenWidth, stage.fullScreenHeight ); 

var w : Number = screenWidth / Capabilities.screenDPI;
var h : Number = screenHeight / Capabilities.screenDPI;
var screenSizeDiagonal : Number = Math.sqrt( w * w + h * h );
var isTablet : Boolean = screenSizeDiagonal >= TABLET_SCREEN_SIZE; // TABLET_SCREEN_SIZE = 6

Please note that this is just a snippet of the full code I use, and there is actually a whole bunch more that goes on to detect proper screen size and orientation.  This will work in most cases though.