The nutrition information panel
UK food packaging (and most European packaging) shows the energy, fat, saturated fat, carbohydrate, sugars, protein and salt content per 100g and per portion. The per 100g column is most useful for comparing products; the per portion column is more relevant if the stated portion is realistic.
Traffic lights
Front-of-pack traffic light labelling — showing red, amber or green for fat, saturated fat, sugars and salt per portion — gives a quick overall indication of nutritional quality. A food with multiple red lights is high in fat, salt or sugar; green across the board indicates a healthier choice. The system is voluntary (not mandatory) but widely used by major retailers.
The ingredients list
Ingredients are listed in descending order of weight. In practice this means: if sugar is the first or second ingredient in a product presented as healthy, the product is likely high in sugar regardless of other claims. If you cannot pronounce the first few ingredients or do not recognise them as foods, the product is likely highly processed.
The claims minefield
Nutritional claims on packaging are regulated: "low fat", "high fibre" and "source of protein" have defined criteria. However, "natural", "free from artificial colours", "pure", and similar claims are largely unregulated marketing language. "No added sugar" means no additional sugar was added in processing, not that the product is low in sugar overall — fruit juice can carry this claim while containing substantial amounts of naturally occurring sugar.