Check out the Azar grammar series. There are three textbooks -- beginning, intermediate and advanced -- with clear diagrams, charts, examples and lots of exercises. They have accompanying workbooks available, as well. I have enjoyed teaching with those books over the years.
You can also say that " something is going to happen "in the future. For example: think of a man who is walking without looking where he is going we can say that he is going to walk into the wall . When we say that 'something is going to happen' the situation now makes this clear. As for your question I'm going to throw up the situation now makes it clear that you are going to throw because you're feeling nauseous or you don't feel well you can count these as the evidence . I hope it is clear now.
