It always depends on the person. Just ask away! Where he/she is originally from, how was his/her childhood there, how did things change when the nazis came, what about friends, family, classmates, was he/she in a concentration camp, and does he/she want to talk about it? What was it like to get out of that situation, did he/she wanted to come to this destination, do they feel at "home", or is their old home "home", did they ever go back to their old country and if, how was it different?

There are so many questions, but mostly they will tell you what they want you to tell. Just listen and the questions will come from alone.

I wish you best of luck!