Impolite questions are those personal questions that you ask when you actually do not know them well.

How old are you?
How much do you earn?
Are you married?
Do you have any children?
Do you have any religion?
What political party do you belong to?

Polite quesitons are those you ask following discussion when the foreigner is showing interest in communicating with you.
Polite questions:
age: following his/her disclosing his job, you may comment: it is very impressive because you are already a manager in your 20s! (even though he looks around 30). Or a director in your 30s!
salary: Usually do not ask about his salary. If you have to, go with: "in China, the income tax is very high. How much income tax do you have to pay last year?" then go back and check.
marriage status: If you have to ask that question, you may try looking at his ring finger first instead of asking. Or comment: In China, married people do not necessarily wear rings. Some unmarried people may also wear a ring on his unnamed finger. It may solicitate his answering.
children: Try pointing to a kid or kid's photo nearby: "that is really cute. Well, I have a kid of 5." If he is interested, he/she will follow the discussion.
Religion: Begin with: Well, the recent earthquake/typoon/... tragedy is really heart-broken. Many people lost their family members. In those disasters I do hope there is a god who will protect people... Whatever his/her comment is, you may have a clue whether he had a religion and what it may be.
Political party: just do not ask the question unless you know enough to generate a meaningful discussion between you two.

All in all, do not pop the question from nowhere. Talk first, observe, and solicit the answers. In that way, most of the topics are actually talkable..

Agree. It depends on how close you are..