private var monitor:URLMonitor;

// This is just an example of a function called after application creation complete
private function handle_creationComplete():void
{
	monitor = new URLMonitor( new URLRequest("http://www.someaddress to check against") );
	monitor.addEventListener( StatusEvent.STATUS, handle_status );
	monitor.start();
}

private function handle_status ( event:StatusEvent ):void
{
	trace(monitor.available);
}

Adobe AIR - Detecting network connectivity

actionscript

ActionScript 3