First of all I must say this is very good movie. Truly masterpiece. Ending is faboulos. Twist is fantastic.
But, frankly there are almost none hints to viewer to guess twist. It was almost impossible to predict ending.
I fell little bit cheated here...
There are at least two hints in retrospect: the flashback that shows how the detective lost his wife (why would they spend time on his past if it's not important?) and the fact that Clara is seen crying after the long night of confessions with Alex.
shareYou shouldn't feel cheated, there are actually many hints (even right from the start) throughout, try to re-watch and you will pick them up.
shareThe hints are there as they show in the flashbacks at the end, but at the time they're shown, they lead you to a different conclusion. For example, when they flash back to the detective's car crash, you think it's showing why he's suspicious of Ulloa (because he was sad when his wife died, whereas Ulloa was like a robot - actually that exchange reminded me of Camus' The Stranger).
Overall, at first i was thinking it was between her being alive vs her being a ghost. Then as the story progresses, it becomes more real - it's then between her being alive and her being alive with an accomplice, so who is the accomplice? Right before the reveal, it's dueling accomplices - with the detective claiming Ulloa is the accomplice, whereas Ulloa thinks it's Javier Alonso, the private investigator he saw in the restaurant.
This was really a masterpiece of mystery story telling.
I can't wait to watch it again to see if I can see how early the clues reveal themselves.