How do I find out which template was used to make the game?

[bx83]

It's definitely not sierra - but that's all I remember.
What script file does it affect? Is it viewable, changeable?

[Crimson Wizard]

Template is simply a starting set of resources. You change it by changing resources. There may be a way to deduct the template by analyzing parts of script or sprites or GUI that remained intact, but if you replaced everything then there's no point.

Note: some templates leave "template.txt" in the game folder.