What part of the sentence is "you" in "telling you who that is"?
English is a language of verbs, so we first identify the verb to answer your question.
The verb is "telling."
With a transitive verb (as in this case), the direct object answers "what" or "whom." Now we ask, "Telling what?" The answer, "who that is," is the direct object.
The indirect object answers the question "to what" or "to whom." Now we ask, "Telling to whom?" The answer, "you," is the indirect object.
Putting this all together, we find that "telling" is the verb, "you" is the indirect object, and "who that is" is the direct object.