A good purple dress has a bit of weight to it before you even hold it up against yourself. Not stiff, just substantial, the kind of fabric that drapes rather than clings or crumples the second you sit down. Hold the hem and give it a shake. If it springs back into shape, that's a good sign. If it stays wrinkled like a crisp packet, it'll look tired by lunchtime no matter how flattering the colour is.
The difference between a purple dress that earns its spot and one that ends up at the back of the rail usually comes down to the same things it always does. The fabric should feel like it has some body to it, a cotton blend or a proper jersey rather than something thin and see through that needs a slip underneath just to leave the house. Check the seams, especially under the arms and along the zip, because that's where cheap dresses give themselves away first, with loose threads or puckering that no amount of ironing fixes. Zips and buttons should feel solid in your fingers, not like they'll jam the second you're running late. And the colour itself matters more than you'd think with purple specifically, since it ranges from something close to navy right through to a pink heavy lilac, and the shade in the shop light is not always the shade you get at your desk or on a Tuesday pub lunch.
Before you buy, check it in daylight if you can, near a window or by the door, because purple shifts more than most colours under artificial light. Look at the lining too if there is one, since a cheap lining can pull the whole dress a slightly wrong tone. Check the stitching at the hem is even all the way round, not just at the front where you'd naturally look first.
Purple can be an easy colour to wear, but it does clash with certain things without much warning. Avoid pairing it with reds or oranges unless you really know what you're doing, it tends to fight rather than complement. Patterned tights in bold colours usually do more harm than good too. Where it works well is against plain things people already have hanging up, a black coat, a grey cardigan, white trainers, a tan belt or bag. Gold jewellery lifts it nicely, silver can sometimes make it look a bit cold and flat depending on the shade.
Next time you see one, check the colour in proper daylight before anything else.