I have found that code for you, the variable is simply _url, this will return the address of the swf as a string e.g. 'http://www.yoursite.com/flash.swf'. I have also found an example of how to use this for your flash file.
Code:
myURL = _url;
okDomain = "www.electrotank.com"
if (myURL.indexOf(okDomain) != -1) {
//Goto game or whatever
} else {
//Goto warning screen
}
This just simply checks to see if the okDomain is in the url and respondes acordingly. In the same place as this i have also found a quote "There is nothing you can do to prevent people from taking the SWF file". Hope this helps.
The code was found in Jobe Makars 'Macromedia Flash MX Game Design Demystified' which i would recomend for anyone just starting to code in flash, full of very good examples.
Flabby Rabbit