A quick rule for the breakfast cereal aisle: at least 4 g of fibre and less than 8 g of sugar per serving.
That exact pair shows up in Taste of Home and on a bunch of US dietitians’ sites. I couldn’t find any official body behind “4 and 8” specifically, but the serious references land close: Harvard Health says 3 g of fibre or more (5+ is better) and 9 g of sugar or less; Cleveland Clinic says at least 3 g of fibre and keep sugar in single digits.
So treat it as a rule of thumb, not a law. Good enough to decide in ten seconds in front of the shelf.
The catch is the “serving”. It’s whatever the manufacturer declares, and it changes from box to box (3/4 cup here, a full cup there), so two cereals with the same numbers may not be comparable. And if you pour yourself two servings, the sugar doubles too.
More on eating in general: Guía nutrición.