We can use will for both past and future assumptions. But how do we differentiate if it refers to past assurance or future possibility? For example:
You will know all about Rachel, of course.
Now this conveys two possible meanings 1.) It is a past assumption and the speaker implies that the person knows about Rachel for sure. 2.) It could also mean that the person will know about Rachel at some point in the future.
Another example:
He will have finished eating supper
now.
So how do we know if it refers to the past or the future? Do we need to change our intonation while speaking?
Best Answer
If I am given a piece of paper with the words You will know all about Rachel, then I cannot be sure if the writer is referring to my present or future knowledge of Rachel. So, the sentence by itself is ambiguous. And you could construct numerous similar examples based on the inhererent ambiguity of the various verb tenses (including modals) in decontextualised messages.
But everyday communication is set in a context and such ambiguity is rare. In the case of your example it is difficult to imagine a natural context where any ambiguity could arise. As to the intonation, if you stress will then it can refer only to your future knowledge of Rachel. For example: