like ele said. just ask the person what their previous name was. if they do then cool, you know who it is now. if they dont tell you then they clearly dont want you to know. difference is by asking the person, they at least have a choice to tell you.
im perfectly fine with people knowing what my old name was since i changed it just because it was terrible. Im pretty sure if i had to change my name because of some heat i attracted id be reluctant to tell anyone too