Which twin is which?
They look exactly alike to me. How can you tell which is which?
To me there is no confusion as to which is which. (phrase no question mark)

Which is a question, phrase or pronoun

which for questions
Link it to the other 'wh' question words - which, who, what, why, when, where

Which is which? Which one is which? Which is correct? Which one do you want? Which way is the wind blowing? Which witch is which? Which twin is which? Which cake do you want?
Which would you like, tea or coffee?
Which way did they go?
which of: Which of the secretaries did you talk to?

No question mark when not certain about a specific choice:
They’re all so pretty – I don’t know which one to choose.
It was either whisky or vodka – I forget which.
I don’t know which is worse – spending Christmas alone or watching my friends get drunk.

Phrase - which is which
I don't know which is which.
There is no confusion as to which is which.
And that can make it hard to distinguish which is which.
I can't recall offhand which is which.
It's hard to know which is which.
Which witch is which? question
I don't know which one is which.
Which one do you know? 

Direct question: Which is my seat? or “Which is my seat?” she said.
Indirect question: She wondered which was her seat.
She wondered which her seat was.
I asked them which was which
"Which is which?" I asked them
Did he say which hotel he was staying at?
I'm not sure he said which hotel he was staying at.
Which hotel is he staying at? 

which for adding more information and is usually before a comma.
Bogart starred in the film Casablanca, which was made in 1942.
A skilled workforce is essential, which is why our training programme is so important.
He showed me round the town, which was very kind of him.
She says it's Charlotte's fault, which is stupid, and that she blames her.
Anyway, that evening, which I'll tell you more about later, I ended up staying at Rachel's place.

in which case (=if this happens): Of course it may rain, in which case we shall have to hold the ceremony indoors.

which pronoun - add information about the thing just mentioned. It is usually used for things, not people:
These are principles which we all believe in.
You know that little Italian restaurant - the one which I mentioned in my letter?
Is that the film in which he kills his mother?
The death of his son was an experience from which he never fully recovered.
It isn't a subject to which I devote a great deal of thought.

Thanks to Oxford Learners Dictionary
Cambridge Dictionary