Pick up a good pair before you've even stepped into them and you can tell. There's a weight to the leather, a give when you press your thumb into the shaft, and the zip pulls up smooth in one motion rather than catching every inch. Cheap ones feel stiff and hollow, like the boot is holding its shape through cardboard rather than construction.
These are not boots for layering with much below the knee. A midi skirt with any volume turns the whole look bulky, and a short, full skater skirt makes the gap between hem and boot top do something unflattering rather than sleek. They work best with things that get out of their way, a slim dress, skinny or straight trousers tucked in, a longline coat that skims rather than flares. Pile on a chunky jumper and a cropped puffer at the same time and you end up looking shorter and wider than you are, which rather defeats the point of a boot designed to lengthen the leg.
Be honest with yourself about your calves and your days before you buy. If you're on your feet all day, walking a dog twice round the block or standing on a crowded platform waiting for a delayed train, a boot this tall with a rigid shaft can dig in behind the knee in a way that gets worse, not better, as the hours go on. Some of us have calves that need a stretch panel or a wider fit through the leg, and if a pair only comes in one generous width, it's worth trying them sitting down as well as standing, because the fit changes completely once your leg bends. Wet weather is another honest limit. Suede over the knee looks lovely for about one dry afternoon.
What separates the ones worth keeping from the ones you'll bin after a season comes down to a few things you can check in the shop. Real leather or a dense, well backed faux should keep its shape and bend at the ankle rather than crease sharply across the front after a few wears. The zip matters more than people expect, a decent metal one with a backing tab will survive a winter of being yanked up in a rush, where a flimsy nylon one splits by Christmas. Check the sole is stitched or properly bonded, not just glued thin, because that's what tells you whether these will see you through wet pavements for a few years or fall apart after one soggy commute. A good pair still looks right after a season of being folded into a bag for travel, the shaft springing back into shape rather than staying creased, and that alone is worth paying attention to before you buy.