A lot of spoilers for Blade Runner 2049 in this one!
How did K find out at the end that
Dr. Ana Stelline is Deckard's daughter?
Freysa, the leader of the Replicant Freedom Movement, revealed to him
that Deckard and Rachael had a daughter, indirectly implying that he was not the child as he first thought.
He also knew that the memory with the wooden horse hidden in the furnace was real - he found the horse and Ana confirmed it's authenticity.
Ana's tears that told K that she was the person who'd actually lived the memory,
or is there something else I am missing?