Question: do they find out that katherine is in elena's body

Answer: Yes, Damon wakes up chained on the Salvatore house's basement. Stefan is there and Damon tries to warn him that because of his cravings, one day he will kill him but Stefan tells him that they will find a solution. Damon points to Stefan that ``Elena'' provoked him to feed on her and then she kicked a stake towards him so he will kill him. Stefan does not believe that Elena would want that, he locks Damon up and gets upstairs where Caroline is. The two of them discuss what happened between Stefan and ``Elena'' and Caroline also tells him about Nadia and Matt and the text he sent to her. From Matt's text, who used ``K'' and not ``E'', they put the pieces together and they realize that Katherine is in Elena's body.