How do you say "Do you speak japanese"
 

I wanted to say do you speak japanese in japanese but forget if it was
にほんごをはねせますか。
or
にほんごをはなせますか。

日本語がわかりますか？

That's "Do you understand Japanese?"

nihongo o hanasemasu ka?

Do you speak Japanese?
Nihongo wa hanashimasu ka?
日本語は話しますか。

This is correct. 日本語が話せますか is also okay.

Ignore other posts that says otherwise. There are a lot of people who wants to help but sometimes they provide the wrong answer.

I didn't think you could use the 話せる kind of form (potential?... don't know) with を... Or at least I haven't personally seen it like that as of yet, and whenever i've said something like 日本語を話せる i've always been corrected to say 日本語が話せる for example.

Good point.

As far as sheer grammatical correctness, I'd have to say both are correct. But if the question is which sounds more natural or which is heard more often in Japan, then the answer is definitely が.

Nagoyankee, I've heard that using を instead tends to change the emphasis of the sentence, like maybe it emphasizes the "Japanese" (as opposed to another language). In a sense, it makes the ear jump to attention and say "wait a minute, there was supposed to be a が there!" and pay attention to the 日本語 then.

I could have been misinformed, though. It's something I think I saw on sci.lang.Japanese back in the day. I definitely always stick with が because it's safe and I don't need to worry about such subtleties too much yet.

So was I misinformed? I defer to a native speaker's experience on this one.

i thought を　was for actions, and が was for emphasis of the subject or something abstract like a feeling, and existance "います”

日本語 (を) はなします i speak= action ?
日本語 (が) わかります i understand= thought/abstract ?

日本語をはなしますか？　or....
nihongo wo hanashimasu ka?