Then you would want to decompile the swf, either using a program like ******* SWF Decompiler or Flare, or use a program like Flasm which will dump the flash asm to a file. You can then look over these files for the variables you want.

Another way to do it would be to decompress the swf file, and then look through the file for specific string.

There are many approaches to what you want, you could even brute force/dictionary or dictionary attack (I say attack, just the method)

If the file is encrypted, you will have to take another approach (or the BF, but that is a long shot, unless you have a reasonable idea of the variables name)
Other problems may arise with variables that are dynamically generated by the actionscript, as they have no defined name and so therefore cannot be easily found.

The final thing to say, the best approach to this would be to code a variable scanner in actionscript. I've seen examples of these, however, they are all kept under lock and key.