grammar - The use of "will" in assumptions

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?