What episode does the find out Serena is sailor moon?

There's a bit of a misunderstanding here! Serena (or Usagi Tsukino, as she's known in Japan) *is* Sailor Moon. She doesn't *become* Sailor Moon in an episode.

She is Sailor Moon from the very beginning, and the series follows her journey as she discovers her powers and destiny. There's no reveal episode; it's all about her coming to terms with her identity as the "Moon Princess" and the leader of the Sailor Scouts.