For years I thought polka dot was a pattern that only worked on children or people confident enough to stand out in a car park. I had one dress with dots the size of golf balls that made me look like I was about to bowl a hoop down a hill. Then I tried a dress with small, scattered dots on navy rather than black, and realised the problem was never the print, it was the scale and the base colour I kept picking.
The dots clash with almost any other pattern, so don't try to layer in a striped cardigan or a floral scarf thinking it will look playful, it just looks like you got dressed in the dark. Stick to plain accessories and let the dress carry the whole look. I'd also steer clear of anything else spotty nearby, two polka dot pieces in one outfit reads as a costume rather than an outfit, even if they're different sizes of dot.
The difference between a good one and a disappointing one usually comes down to fabric and how the dots were made. A dress with dots printed onto a cheap polyester can look slightly blurred at the edges once you're close up, and the fabric itself tends to cling and go a bit shiny by the end of the day. A viscose or a proper cotton poplin holds the print crisper and drapes instead of sticking to you on a warm bus. Check the finish at the hem and the buttons too, a dress that's had care taken over matching the pattern at the seams will have dots that line up rather than suddenly jumping half a centimetre to one side, which is a small thing but it's always the small thing that gives a cheap dress away.
For fit, polka dot does you a favour because the eye is drawn to the print rather than the seams, but that only works if the dress fits properly to start with. Go for your usual size and check the waist seam sits where your actual waist is, not an inch below it, because on a lot of these dresses the waist is cut high and if you're even slightly longer in the body it ends up sitting under your bust instead. Watch the sleeves too, puff sleeves in a heavier cotton can feel stiff and architectural rather than soft, so if you're between sizes go up, nobody wants a sleeve seam cutting in at the top of the arm all afternoon.
A good one has a slight weight to the fabric so it swings properly when you walk, rather than floating up and sticking to your legs.