How can I know COE was accepted?

I called immigration and they said they are preparing a letter so they will send me soon.
Does that mean I will get COE or rejected?

You cannot guess anything. After receiving the post only you will know. When and where you applied for COE and what kind of residence status you applied for?

My wife applied for my COE in osaka.
It's for spouse visa.
My wife said it's surely approved because when she called them to check they said so, but i have doubts.
They said the procedure is finnished and are going to prepare letter and send it soon. She said that it's approved.
I am not sure about that...

if your wife is a Japanese, she was able to get some information (of course, it is indirect.) from the conversation.
in Japan, it is traditional and common that no one says an answer by phone call, because it often makes troubles. a clear result is given by a document sent by mail.

I also think it's hard to get clear answer from immigration, but she keep saying that it is positive answer.
She already calles twice. First time they said it's going well and it's in finale process.
When she called again they told her the procedure is finnished and they will send the letter soon, we just have to wait some more.

So I do not know if i should feel happy already or nervous.