Script Debugger lets you see invisible text characters, such as tabs and return characters. To see invisible characters:

• Choose View > Show Invisibles. If the menu item is checked, invisibles are showing.

This is particularly useful for viewing string literals. AppleScript has an annoying habit of turning whitespace literals into literal whitespace, which conceals what they are. For example, this script:

  set s to "\t\r\n"

compiles like this:

  set s to "    
  "

Thanks a lot, AppleScript! Now you’ve no idea what that string consists of. But with Script Debugger’s Show Invisibles feature, you can instantly find out.

This is particularly useful, as in the above example, with line endings, which can present a complicating factor in AppleScript, as explained here.