The giveaway isn't the length, it's the proportions. A tall dress that's just a regular one with extra fabric chucked onto the hem looks fine on the hanger and wrong on you within a week, because the waist sits too low, the sleeves stop short of your wrist bone, and the whole thing starts to look borrowed rather than bought. You notice it most in photos next to someone shorter, where suddenly your proportions look slightly off and you can't work out why. That's the fabric telling on the pattern.
A well made one is cut from the shoulder down with the extra length distributed through the torso and the skirt, not just bolted onto the bottom, so the waist seam sits where your waist actually is and the sleeves reach your wrist rather than your forearm. Feel the fabric before anything else: a tall dress in something with a bit of weight to it, a proper midweight jersey or a cotton with some structure, will hang straight down and skim rather than cling, where a thin, slippery fabric rides up and clings to the wrong places by lunchtime. Check the seams at the shoulder and underarm, because that's where cheaper versions pull first, and look at the hem, a deep double stitched hem drapes better and lasts longer than a narrow overlocked one that curls after a few washes. These pair easily with things you already own, flat boots or trainers rather than heels if the length's already generous, a denim jacket thrown over the top for the school run or the commute, a belt if the dress is a looser shape and you want a waist to appear out of nowhere. Over months, a good one keeps its shape through the body and just softens slightly in the fabric, which is honestly the best outcome, worn in rather than worn out. Wash it cold, hang it rather than tumble dry it, and it'll still fit the same in a year, which is not nothing when you've spent actual money trying to find length that doesn't need altering.
If you're starting with none, go for a plain jersey shift in a colour you already wear, nothing with a print that draws attention to length you're trying to make look normal. Get the proportions right once and every other tall dress you buy after that gets easier to judge.
Thrown over leggings or layered under the jacket you never know what to do with, it finally looks like it was made for you, not adjusted for you.