I\'d like for a script of mine to have 2 behaviours, one when started as a scheduled task, and another if started manually. How could I test for interactiveness?
You should simply add a command-line switch in the scheduled task, and check for it in your script, modifying the behavior as appropriate. Explicit is better than implicit.
One benefit to this design: you'll be able to test both behaviors, regardless of how you actually invoked the script.