Stand in front of the stack and do the hand test before anything else. Good linen feels slightly rough right out of the package, almost papery, and that is correct, not a flaw. It softens with washing, not with some chemical finish that makes it feel soft on day one and pills by month three.
The difference between linen that lasts and linen that disappoints comes down to the weave and the weight, and you can mostly tell by holding it up to a window. A lower weight, loosely woven set will feel nice in the store and then go limp and see through after a few washes, while a denser weave holds its body and actually gets better with age, developing that slight crumple that makes a bed look lived in rather than ironed into submission. Check the seams too. Flat, even stitching along the duvet cover opening and corner ties that actually line up with the corners of your insert matter more than thread count ever will, since linen doesn't play that game the way cotton does. Stonewashed or enzyme washed are the finishes worth looking for on the label, since that's what gets you the soft, slightly rumpled hand feel without stripping the fiber's strength. A set that smells faintly of nothing and feels stiff but not scratchy is a good sign. One that already feels fuzzy or thin is not going to hold up to a toddler, a dog, or just regular Tuesday night sleeping.
Where it falls short is temperature extremes and anyone who wants a bed that looks made the moment they pull the cover up. Linen breathes beautifully in a warm room and feels cool against skin on a July night, but in a drafty old house in January it won't hold heat the way a flannel or a heavier cotton sateen will, so you're layering a blanket underneath regardless. And if you like a crisp, hotel corner look, this isn't your fabric. It wrinkles on purpose, and no amount of effort unwrinkles it for long, so anyone chasing a smooth, styled bed is going to fight it every morning.
In an ordinary bedroom it sits best against warm wood tones, worn rugs, and morning light rather than bright overhead bulbs, since that's when the texture actually shows instead of just looking creased. For sizing, measure your mattress depth with any topper already on it, not just the mattress alone, because a shallow fitted sheet on a tall bed is a nightly fight you will lose at 2am. Get a cover sized exactly to your duvet insert, not approximate, since linen has almost no stretch to forgive a mismatch.
Start with a soft, faded white or a warm oatmeal. Easiest to live with, and it hides the wrinkles best of all.