I agree with Ryoko-san. If you look like a person who are living outside Japan, many people here would say 日本語、話せますか？ because they think it's natural that you don't speak Japanese. If you look like a person who it's unnatural to speak Japanese in some reasons, they would say 日本語、話しますか？, but it sounds unnatural a bit. I would say 話していますか？ as Ryoko-san said. Or some people say 日本語、話しますか？ to anybody because they know some foreigners who know Japanese a little would feel bad when they are asked 話せますか (Can you ～？). Those foreigners insist that "Can you" is rude because they feel the speaker would think they have no ability. (I think it's ridiculous, though. When people here say 話せますか, it doesn't have such a meaning)

I would ask 日本語を話せますか to the people from overseas when we see and talk for the first time. If there are someone who is married with a Japanese or who lives with Japanese and I want to know which language they speak at home, I would say 日本語を話しますか / 話していますか. I hope this was helpful.