In OP's context, the future tense verb form doesn't necessarily imply any direct reference to a future time. For example, there are several hundred written instances in Google Books of...
[He] will be asleep now
...where obviously the word now implies present, not future time. But it's important to note that native speakers often use He would be asleep now in exactly the same context, with exactly the same meaning.
In my example, you could interpret will as implying something along the lines of it will become apparent [in the near future], and would as implying the irrealis/subjunctive if it were to be definitively established.
But in practice I don't believe native speakers think that way. It's just that using the "future" tense places a bit more "distance" between the speaker/time of speaking and the referent. For example, Stanley's famous greeting could have been (but wasn't, I believe)...
"You will be Dr. Livingstone, I presume?"
Depending on the exact context, the precise nuance imparted by introducing You will be in such a construction may vary. For example, it could convey respect (you are different/more important than others in the presently-assembled company), or disrespect (you are different/less important than them).
Having said all that, the use of future tense in OP's exact context usually implies you believe or expect your statement will be verifiable. If Ann had only just told you she'd be busy for at least the next hour, you'd use present tense (because you know she is busy).