You can spot a good white jacket from the other side of a room. The good one sits clean and bright against everything else, the fabric holding its shape at the shoulder and the white staying white, not greying at the cuffs by lunchtime. The poor one looks slightly grubby before you've even put a coffee near it, thin enough that you can see the seams and the bra strap underneath, and it creases the second you sit down in a car.
White jackets are fussy about company. Dark denim that hasn't been washed enough will leave blue rub marks on the inside of the cuffs within a day, and newspaper print or anything with loose dye, like a cheap tote bag or a fresh leather handbag, will transfer the moment it brushes past. I keep mine away from anything that's just been dyed or printed, and I think twice before wearing it anywhere with a lot of red wine involved, which in my case is most places.
Fit matters more on white than on any other colour, because every pull and gape shows up as a shadow line. Go for your usual size and check the shoulders first, they should sit right at the edge of the bone, not hang off it or dig in. If the jacket is cut close, a tight button across the bust will show as a cross hatch of strain lines under the white fabric, so size up rather than squeeze in. Sleeve length is the other thing to watch, since white shows a dropped cuff edge far more obviously than navy or black ever does.
The difference between a decent one and a disappointing one usually comes down to the weight of the fabric and what's underneath it. A good white jacket uses a weave with a bit of body to it, so it doesn't go sheer under shop lighting, and it's lined properly so the inside doesn't go grey with wear before the outside does. Check the buttons too, proper horn or metal ones sit flush and don't yellow, while the flimsy plastic ones on cheaper jackets tend to discolour long before the fabric does. Stitching along the hem should be flat and even, not the wavy, puckered line you get when a factory's rushed it.
It isn't the jacket for anyone who commutes by bike, does the school run with a toddler mid ketchup phase, or works anywhere near a printer that leaks toner. It's also not forgiving if you're someone who likes to throw a jacket on without checking what's happening underneath, because every seam, every bra line, every bit of static cling shows through white in a way it simply doesn't on darker fabric.
Worn with care, a white jacket looks more expensive than almost anything else in the same price bracket, and that's not nothing.