'Who you telling' is an informal shortening of 'who are you telling', used to mean strong agreement, like you're telling me or I know. It sounds like a question, but the speaker is not asking anything; they are saying they understand completely because they have lived the same thing. For example:
My landlord raised the rent again. Who you telling? Mine did the same thing last month.
Don't take it as a literal question about whom the speaker is talking to. It is simply an emphatic way of agreeing, most common in casual speech and African American Vernacular English.
This bus is always late. Who you telling? I've been waiting half an hour.
It isn't a real question, so no answer is needed; the speaker is agreeing from experience.
My boss keeps adding work on Fridays. Who you telling? I stopped making weekend plans months ago.
You're telling me. is the standard version of the same idea. It shows a thing is so clear it need not be said, or shows strong agreement. The difference is only formality: who you telling? is the shorter, more casual cousin of you're telling me.
Don't take it as a literal question about whom someone is speaking to. If you answer I'm telling you, you've missed the meaning; the speaker is agreeing, not reporting.