I do this with punctuation. I tell my students that by default, I'll use exclamation marks when empty boilerplate phrases regardless of my actual enthusiasm, e.g. "Sounds good!". For students whom I am more familiar with and don't need the fakery, I'll settle for "Sounds good.". But depending on how petty and irritable I am at the moment, maybe the closing period will be used as a passive-aggressive way of implying, "No, that does not really 'sound good'.", and you'll just have to guess!

After awhile, students appreciate the way programming language interpreters immediately and decisively raise errors on even the simplest typos and syntax errors.