For years I thought a leather boot was a leather boot, and the only thing that mattered was whether the colour matched my coat. I bought two pairs that cracked across the toe within a winter and decided leather boots just weren't built for Birmingham weather. Turns out the weather was never the problem, the leather was, most of it a thin coated finish standing in for the real thing.
In the shop, before anything else, I check the inside of the boot, not the outside. Pull back the tongue and look at the lining, is it a proper textile or leather, or does it feel like it's been glued in as an afterthought. Press the sole where it meets the upper, a well made boot has that joint stitched or securely bonded, not just sealed with a bead of adhesive that will let go the first time it rains properly. Check the zip pull is metal, not a plastic tab dressed up to look like it, because that's the part that fails first and takes the whole boot down with it. And always check the heel is attached evenly, I've had one pair that sat slightly skewed from day one and I ignored it because the price was right, which was my mistake, not the boot's.
The difference between a boot that's worth the money and one that isn't shows up about six weeks in, not on the day you buy it. Good leather has a bit of give to it, it creases naturally across the foot rather than cracking in one sharp line, and it takes polish or cream without going patchy. Cheap leather looks fine under shop lighting and then dries out fast, especially around the toe where your foot flexes most. The sole matters just as much, a chunky moulded sole that flexes with your step will survive a winter of pavements, while a thin smooth one looks elegant in October and is see-through by January. Stitching down the seam tells you a lot too, even, tight stitches mean someone cared about the construction, loose ones mean they were racing to hit a price point.
Where leather boots fall short is anywhere properly wet or properly cold. Even good leather needs reproofing, and if you're standing at a bus stop in sleet most mornings, a treated synthetic boot will outlast it without complaint. They're heavier in a bag than people expect too, not something you're throwing in on the off chance. What you want, the very first thing to check when you pick one up, is that seam where the upper meets the sole, run your thumb along it before you look at anything else.