The first thing I do with a parka is grab the hem and give it a tug, because that tells you more than the label ever will. If it swings loose and flappy, it's going to feel like a tent by October. A good one has a bit of weight to it, a drawstring or a shaped waist somewhere, something that stops it looking like you've borrowed your dad's decorating coat. I've owned both kinds. The flappy one got worn twice and spent three years hanging behind a door, taking up space, making me feel guilty every time I saw it.
It's not a coat for everything, and the sooner you accept that the happier you'll be with it. It struggles in proper cold, the kind where you want down and a high collar, because most parkas are built for drizzle and wind rather than minus temperatures, so layer underneath if you're wearing one past November. It also does nothing for you if you need to look put together quickly, it's a throw it on coat, not a tailored one, so for anything smart leave it on the peg. Where it's unbeatable is that horrible in between weather, the grey weeks either side of winter when it's not cold enough for wool but too wet for a jacket, school runs, dog walks, standing at a bus stop wondering why you didn't check the forecast. That's its whole reason for existing. The difference between a parka that earns its spot and one that disappoints is almost always in the details rather than the main fabric. Check the zip pulls first, cheap ones snag and then stick, and you end up wrestling with it in the rain looking like an idiot. The hood matters more than people admit, a wired peak keeps it off your face in wind, a floppy one just blows back and slaps you. Look at the lining too, a proper quilted one keeps you warm without adding bulk, whereas a thin nylon lining does nothing and rustles like a crisp packet every time you move. Toggles, poppers, storm flaps, all small things, all the difference between a coat that lasts five winters and one that looks tired by February.
Where it falls apart is pairing it with anything delicate, silky trousers, heeled boots, a nice bag you're precious about, because the parka's whole personality is practical and it will drag anything fussy down to its level rather than the other way round. Wear it with trainers, jeans, something you don't mind getting rained on.
Buy it roomy enough to fit a jumper underneath without the zip straining across your chest.