I hear you. It's very tough to get constructive criticism as a manager. I've found you generally force others to answer more than just "how am I doing", but specific situations "how did I handle that". You'll have to interpret their answers, because often your reports especially will not want to give full answers. But you can get closer over time as you build trust and when you take slight "negative" feedback that you take it well and you don't retaliate, than they will give more stronger, closer to the truth feedback.

Its a journey, for sure!