How to be 100% sure that this vil means will and not want?
Context. However it would in most cases mean 'want to'.
It just means want, not will... ex. I want to give you something to drink.
If you want to say "I will" you say: Jeg skal...
The Skav and Vil section of Tips and Notes for this Skill seem to indicate otherwise. Are the tips and notes here incorrect?
I'm not quite sure if it's incorrect, but it sounds unnatural. My mom is a norwegian teacher and she looked over my previous comment, so my information should be correct.
Gotta disagree with you mom there. "Det vil bli en fin dag" means "it will be nice day" for example.