Asking and listening (imo) will not give you accurate reflections of their true traits. Test them like any other investor would test you for fund-worthiness.
Find out what else they've done to demonstrate they are entrepreneur material. Do a short trial project or two. Pay close attention to how they react to different circumstances during the projects. Challenge their ideas and see how they respond. Ask yourself if you could really work well with them, and if they really improve the odds of success enough to merit becoming a significant shareholder.
Ask them to tell you stories about themselves that exemplify these traits.