It depands, what type of past facts you want to know, ikiwa utamuuliza alikuwa na wanaume wangapi, umelala na wangapi, ulikwenda bar gani and to do what, these are dangerous questions na ndio ikawa mwisho wa relationship yenu.
There are things you can discussed but (for me) is not good idea kuuliza past relationship unless, unless she/he start the topic, still you should not ask wanangapi walimbinjuwa/ulibinjuwa.
There are numbers of romantic questions ambazo utaweka relationship yenu healthy.