Across a room you can spot the difference in about two seconds. A good white bag has structure in the base, a clean set to the leather or coated canvas, and it holds its shape whether it's full or practically empty. A poor one goes slightly grey and crumpled at the corners within a month, sits on a table like a deflated balloon, and the white itself looks chalky rather than clean. Once you've seen both side by side you stop trusting a photo and start checking seams instead.
Where it falls short is obvious enough but worth saying anyway: this is not the bag for a wet commute, a toddler's snack bag, or anything involving red wine near a white sofa, because white shows every transfer mark from denim, newsprint and other bags rubbing against it in storage. Smooth leather and leather look-alikes mark more easily than you'd expect from a handle rub or a car seat buckle, though a cotton or canvas structured tote in white is more forgiving day to day and wipes clean with a damp cloth rather than sulking about it. Over months, a decent one ages into a soft cream rather than a dirty grey, especially if you keep it stuffed with tissue paper when it's not in use so it doesn't collapse at the sides, and you wipe off any marks as soon as they happen rather than weeks later when they've properly set in. A protective spray made for the material helps before the first outing, and a slightly dark lining is a genuinely practical choice because a pale interior shows pen marks and make up residue almost immediately. This kind of bag suits spring and summer most obviously, against pale dresses and tailored trousers, but it also does something sharp against black in fabric winter coats, which is worth remembering when you're tempted to pack it away from October onwards. For occasions it sits nicely between smart and casual, good for daytime events, lunches, weddings in warmer months, anything where you want the outfit to look considered without trying too hard.
Before you buy, check the stitching along the base and handles for any pulling, press the material to see how easily it creases, and look at the zip or clasp hardware up close, since a cheap zip on a white bag is the first thing that will look tired. Hold it open and check the lining colour and whether the bag stands up unsupported on a flat surface, because that tells you everything about how it will behave in six months.