For now, until there is another option, here is a quick way to tell if the application you are looking at was created in Flex or Flash.
Right click on the swf and select “zoom in”. Right click again and click zoom out. If it goes back to the original size then it was created by Flash 8 or earlier. If you zoom in and then zoom out and it does not zoom out all the way then it is Flex 2. This trick may not work in future builds obviously because it is a bug.
Alternatively you can usually tell by the theme. The look and feel of the components are slightly different between Flash and Flex. Flex components can be seen here http://weblogs.macromedia.com/mc/archives/FlexStyleExplorer.html. They also have different tween types. You can see examples in the Flex Style Explorer.
I put in a feature request to show “Created by [App Name]” in the context menu where App Name would be replaced by either Flash or Flex. If you want the same send it to the wish list. We’ll see if it materializes or not.