For those short of time, six points.
One: ask the percentage of down. Ninety per cent or more.
Two: ask the class. Class I of EN12934.
Three: ask the fill power. Above seven hundred.
Four: ask what the casing is made of. Natural fibre, tight weave.
Five: ask whether tailor-made volants are possible for your bed.
Six: ask what happens after five, ten, twenty years. Washing, refilling, mending, does that come with the purchase?
Whoever can put these six questions and get a clear answer, buys well.