So to make this sentence a pronoun case orginal sentence is Have you guessed the identity of the person to who I am speaking?

correction is Have you guessed the identity of the person to whom I am speaking?

Correct.

To understand the correct pronoun case in this sentence, we need to determine the appropriate usage of "who" and "whom."

Here's how we can break it down:

1. Identify the main verb: The main verb in this sentence is "guessed."
2. Determine the subject: The subject of the sentence is "you," the person who is doing the guessing.
3. Identify the preposition: In this case, the preposition is "to" because it shows the relationship between the verb "speaking" and the object of the preposition.
4. Identify the object of the preposition: The object of the preposition is "whom I am speaking." This is the person to whom the speaker is currently speaking.
5. Choose "who" or "whom" based on its role in the sentence:
- "Who" is used as a subject pronoun.
- "Whom" is used as an object pronoun.

In the given sentence, "the person to [whom] I am speaking" functions as the object of the preposition "to." Therefore, we need to use the object pronoun "whom."

The correct pronoun case is "to whom I am speaking."