What kind of questions are too personal to ask people?

I am the kind of person where you can pretty much ask me anything and I will gladly give you an answer. This makes it difficult sometimes though because I don't know when a question is too personal to ask someone else? It seems to vary with different people. I have some friends for instance who are completely open about their sex lives and others who aren't at all. But with people you don't know very well it's hard to ask questions like that. So does anyone have some broad advice on what is appropriate to ask different kinds of people and how to tell if they are the kind of person who would get offended or not. Specifically pertaining to questions about relationships and sex. Thanks!