Who or Whom Again

Kay asked:
Have you guessed the identity of the person to who I am speaking ? (Pronoun Case) What is the correct way to write this sentence?

It should be "Have you guessed the identity of the person to whom I am speaking?"

"To" is a preposition, and pronouns following a preposition are in the objective case:
I am speaking to him.
I am speaking to them.
To whom am I speaking?

Here are some more examples:
From whom did you get this letter?
I got it from her.
From whom was this letter sent?
This letter was sent from him.