A good olive dress has a bit of weight to it before you even unfold it properly, the kind of fabric that drapes over your arm rather than slithering off it. You run a thumb over the seams and they sit flat, not puckered. There's a particular satisfaction in a colour that reads as neutral and interesting at once, and you can feel when a dress has been dyed properly because the shade looks the same in the shadow of the fabric as it does on top, no patchiness, no green undertone fighting with a brown one.
Construction is where olive dresses fall apart, quite literally, more than most colours. Cheap versions use a thin cotton or viscose blend that goes slightly translucent under certain lights, which you will only discover standing near a window, not in the changing room. Look at the stitching along the waist and hem, because a dress that's been finished properly won't fray within a month of normal wear. Hardware matters too, a zip that catches on the second use was never going to make it to fifty. The better ones use a fabric with a bit of structure, a twill or a heavier jersey, so the dress holds its own shape rather than relying on you to hold it for it.
Olive clashes with more than you'd expect. Anything mustard or rust nearby tips the whole look autumnal in a way that can feel costumey rather than considered, and I've learned to be careful with warm browns too, since they flatten olive into something murky instead of letting it read as its own colour. Cool greys and black work, and so does a plain white, but a khaki or a camel jacket on top of an olive dress is one green note too many and it starts to look like you got dressed in the dark.
Over a season, olive is honest about how it's been treated. It holds colour well through washes if you turn it inside out and keep it out of hot water, but the fabric itself will tell on you, pilling under a bag strap, going shiny at the elbow if you've been leaning on tables in it all winter. Hang it rather than fold it for storage, because the colour hides creases less forgivingly than black does, oddly, those lines show up almost silver against olive when the light catches them.
What makes a good one worth keeping is a hem that's been cut on the bias enough to swing slightly when you turn, just enough movement that you notice it without meaning to.