The first thing I check is what happens when I pull the fabric against itself. Cheap nightdresses cling and crease and go a bit see through under a bedside lamp, and you can tell that before you even try one on, just by scrunching a handful in your fist and watching how it falls back. A good one drops. It has weight to it even when it is thin, which sounds like a contradiction but isn't once you have felt the difference between a proper cotton lawn and the synthetic stuff that feels cool to the touch for about four seconds and then doesn't.
Sizing on nightdresses is strange because the number on the label barely tells you anything useful, it is all about where the fabric sits and how much room it gives you when you roll over in the night, which you will do a lot. Go a size up from what you'd wear in the day if the shape is fitted through the body, because anything that nips in at the waist will ride up around your middle by three in the morning and you will wake up with it bunched somewhere near your ribs. Loose, shift shaped ones are more forgiving, but check the armholes, a lot of them are cut too high and dig in if you have any kind of bust at all. Length matters more than people admit, knee length looks neat in a photo and then flashes everything the second you sit on the edge of the bed, so if you actually want coverage, buy longer than you think you need. The fabric that separates the good from the disappointing is usually in the detail you'd never think to look for first, the hem. A rolled or double stitched hem survives a hot wash and years of being kicked to the bottom of the bed, a single raw edge unravels within a month. Buttons should feel solid, not the flimsy plastic ones that crack in the washing machine, and any lace trim should be sewn flat, not just tacked on at the edges, or it curls and catches on everything within a fortnight. Brushed cotton and modal both wear well and soften with washing rather than pilling, while anything heavily printed polyester tends to go bobbly at the exact spots where the fabric rubs against itself, under the arms and across the hips.
If you have none, start with something in plain cotton, mid length, a simple cut with no fitted waist, nothing you have to think about once the light is off.
Would I still want to be wearing this one in a year, or is it only good for the first few washes?