When I hear someone knocking on the door, and I think that John is behind the door. should I say "That would be John" or "That will be John"?
Either can be used correctly.
That would be John.
Is basically saying "I assume that that is John" (= an assumption in the present)
That will be John.
Is basically saying "The person who knocked will soon be revealed to be John" (= a certainty in the near future)
Notice the difference about how certain the speaker is. "would" communicates that the speaker guesses that it is John, "will" communicates that the speaker is sure that it must be John.
The speaker can of course still be wrong, but at least he isn't doubting himself at the time of making the statement.