Overweight ball python is rather an oxymoron. But if you ever do end up with an overweight BP, you'll know from the proportions, most notably the skin will be stretched throughout its body where you'll see the skin between its scales, it will often have many FAT folds where its body bends, the head/neck will look proportionally too small and the tail will look very distinct from the body, as if it were just plugged onto a sausage, as opposed to the natural streamline appearance. The side patterns may also be pointing upwards from the expansive fatty tissues, and the snake's girth may have a very blob-like oval shape when laying flat as opposed being perfectly round and firm. An obese BP will essentially look ready to explode with white fatty tissue, just like any other obese animal. You'll know an obese BP when you see one.
But if you're feeding a typical weekly schedule of appropriately sized meals, then there is most likely no reason for concern, and it's rather difficult to overfeed a BP to the point of being fat.
Skin will make folds when they're bending extensively and coiled, and that's normal.