I would ask X-san (X being a name) what to call him or her as follows: Xさんのことをなんと呼んだらよろしいですか？ (X-san no koto wo nanto yondara yoroshii desuka?) I hope this is right! :)

I wonder if I didn't get your question for my English. Do you know "how you can call the family members of your host family"? If so, I would say: (for all of them) みなさんのことを なんて よんだら いいですか。(for one of them whom you talk to) (あなたのことを) なんて よんだら いいですか。If I understood wrong, please let me know.