My advice is taliking to them like ｢しつれいですが、にほんじんですか?｣ means Excuse me but, are you Japanese? Then you will get the answer in their language.

In all honesty, if it's within the US, befriend them first, then ask in English. A lot of Asian-Americans of my generation and younger are American born and have little compunction/means to speak the language of their forebears (I'm a case in point, albeit an Asian-Briton, hell one of my best mate's grandfather looks as Chinese as you can get but once he opens his mouth... ho, boy! Yeah, he's West Indian, but I'm not sure which island) given that they grew up surrounded by largely monolingual Americans, and most Asian-American parents tend to be easy going about the language(s) their kids speak. And those who do speak it (or not, even) may believe such a question (even in English) straight off the bat to be a touch presumptuous, more so than racist (while some won't make that distinction). At least that's my take on it... :| Apologies if this in itself seems presumptuous of what you were after/unduly terse...

After you have some conversation, you will tell them that you are learning Japanese and barely have a opportunity to actually practice speaking it, first. If they were fluent in Japanese, they might talk to you in Japanese or they would continue to speak in English if they were not sure about Japanese. By doing this way, I think you can avoid somewhat conflict. Of course, it is better you have to continue the conversation after you find they were not Japanese. Just for information, when I went outside of Japan, someone told me that the person was learning Japanese and we had a short conversation. But when another person asked me if I am Japanese or not, I felt little bit uncomfortable.

You might check to see if there are any groups in your area that hold Japanese cultural events. That might be a way to find people to talk with, but otherwise, I agree TTsTiTT-san. Make friends and get to know them, as strangers would probably be much less likely to even respond to such questions.

Okay, thank you!! :D