Which verb is used is definitely a decent way to tell whether it's singular or plural! Additionally, "Schlüssel" is a masculine noun, meaning it's der Schlüssel when it's singular and die Schüssel when it's plural. Lastly, there's one thing you usually lack on Duolingo that's very present when actually using the language: context. How can you tell whether the English word "deer" is singular or plural? Context.
Hopefully this helps. ^_^
Well, I've noticed that in most cases nicht goes on the last position, but in every "state of being" sentence it goes right after the verb. And when I say state of being I mean when you use stuff like: ist, sind, seid, or bist. And so, the keys are not big would be, "Die Schlüssel sind nicht groß."
Haha, I hope that wasn't confusing and that it was helpful.