There's a moment in a changing room with a puff sleeve dress where you put your arms down and go "oh" because the sleeves suddenly have a mind of their own, bumping into the mirror, knocking a hanger off the hook. That's normal. It's not a sign the dress is wrong, it's just a dress that takes up more space than you're used to, and you need thirty seconds to work out where your elbows go.
Fit is really about the shoulder seam and nothing else. If that seam sits where your actual shoulder ends, the puff does its job and skims off your arm like it's meant to. If the dress runs small and the seam creeps up towards your neck, the sleeve turns into a stiff little life ring that pins your arms to your sides and you'll be miserable by hour two. Go up a size before you go down one, puff sleeves forgive a looser body far more than a tight one, since the volume up top balances out anything roomier below. Watch the armhole too, cheap ones cut it too tight to save fabric, and that's the fit problem that sends dresses straight back to the rail at work, girls wriggling out of them within about ninety seconds flat.
The difference between a good one and a sad one is almost always whether the sleeve holds its shape without help. Decent ones have a bit of structure sewn in, a stiffened lining or a proper gathered seam, so the puff sits up on its own. Bad ones rely on the fabric alone, usually a thin polyester that looks fine on the hanger and goes flat and crumpled the second you sit down on a bus, then stays that way. Check the gather at the shoulder too, if it's one thin row of stitching it'll pull and pucker within a few washes, a good one has two or three rows holding it steady.
Keep the rest of the outfit quiet, this is not a dress that wants a statement bag or a big necklace arguing with it, plain shoes and a simple cardigan over the top when it's cold do more for it than anything fussy. Skip chunky shoulder bags too, the straps sit right on top of the sleeve and squash it flat within an hour, I've watched it happen to my own dress at a bus stop and felt genuinely wounded on its behalf.
Where it stops working is anywhere you need your arms free, carrying shopping, picking up a toddler, squeezing onto a packed train with your elbows tucked in. Lovely dress, terrible travel companion.