1. How much does one earn? You can obliquely ask what kind of a job he does. That's OK 2. Politics. 3, Asking his what his religion is OK but not giving your opinion about what you think about religions. 4. As a Chinese, most foreigners don't mind being asked about their marital status because they understand your curiousity and to make small talk to learn English. If you are really interested, just look to see if they have a wedding ring on their fingers! Almost western wears it - unless of course purposefully to hide that fact... Then you can make you own conclusions about the person. 5. In general you may ask about his family like how many siblings he has but do not ask too deeply otherwise you may come across as nosy. Of course the above are the general guidelines. How much can you ask depends on the length of time you know the person. When in doubt, ask yourself how would you feel about the other person asking you the same question. Then proceed with your own conclusion.

要看是哪个国家的人吧，具体问题具体分析。不过我觉得无论是对于哪个国家的人来说，宗教信仰都不是个好topic。