For most situations when I want to only execute code of an application running from the editor, I use regular expressions to check the application's url.
You could use other regular expressions for your own situation.

In Flash Builder 4

 
 
                        public static function isDevEnvironment():Boolean
			{
				return mx.core.FlexGlobals.topLevelApplication.url.search(/^file|^\w:\\ig/) != -1;
			}
 

In Flex Builder 3

 
			public static function isDevEnvironment():Boolean
			{
				return mx.core.Application.application.url.search(/^file|^\w:\\ig/) != -1;
			}
 

There are a lot of Flash physics engines available to use. I've only tried Box2D, APE, and Foam.
Foam is my favorite so far. I actually like the options of Box2D better but it's just not as ActionScript-friendly as Foam.

Foam sets up the easiest of these 3 engines for an ActionScript only project...In my opinion, setting it up to use in my Flex projects takes a little more work depending on what I use it for. Seeing the end result of it is fun to watch.

Below I'm currently using my polygon editor to test Foam out in Flex.