When buying a duvet you do not need to know everything. Two questions often bring much to light.
Is it to European standard?
Look on the label for EN12934. That standard gives information about bird, origin, quality class and the ratio of down and feathers. Class I is the highest quality.
What does origin say?
Origin is no detail. Down from cold regions often has better insulating properties than down from warmer regions. Ask about it. A clear answer is reassuring.
Is it craftsmanship?
A craftsman shows what is in the duvet and explains how it is made. He can also tell what happens later: washing, refilling, repair.
Mind the promise
A warranty says something, but after-care says more. And when someone claims real down cannot be washed, something is off. Good down is washed, with care.
Why these two questions can be enough
Standard and craft reveal much. The standard tells what is in the duvet. The craft tells whether one understands what one does with it. Together they give more certainty than a long sales talk.
Mind the attitude
A craftsman shows, explains and takes time. He has no need to hide behind difficult words. When someone shows the down and quietly explains how it is processed, you feel more confidence at once.
A last nuance
As often with good sleep, it is not about one single detail. Material, size, care and feel work together. Only when that whole fits does comfort become natural, and you no longer need to think about it at night.