Front-of-pack design is a marketing surface. The back of the packet is regulated, standardised and boring, which is exactly why it is useful. Three things there tell you nearly everything.
1. The ingredient list, in order
Ingredients are listed by weight, most first. That single rule does a lot of work. If sugar is second in a breakfast cereal, it is a sugary cereal regardless of the whole grain badge on the front. If a "cheese" sauce lists water, oil and starch before cheese, it is a starch sauce with cheese in it.
Watch for the same ingredient wearing different hats. Sugar appears as glucose syrup, fructose, dextrose, invert sugar, molasses, fruit juice concentrate, honey, malt extract, agave. Split across four names, sugar can sit lower in the list while still adding up to a lot.
Allergens are shown in bold or capitals in most of Europe, which makes scanning much faster than reading.
2. Per 100g, not per portion
The per-100g column is the only honest way to compare two products, because portions are chosen by the manufacturer. A "portion" of crisps might be a third of the bag; a portion of cereal might be a quantity no one has ever poured.
Rough guides used by food agencies for solid foods, per 100g: high is more than about 22.5g sugar, 17.5g fat, 5g saturated fat or 1.5g salt; low is under about 5g sugar, 3g fat, 1.5g saturated fat or 0.3g salt. You do not need to memorise those numbers, only the habit of looking at the same column each time.
Salt versus sodium: salt is roughly sodium multiplied by 2.5, so a label listing sodium looks smaller than it is.
3. What the front is allowed to say
Some claims are legally defined and some are decoration.
- Defined: "low fat", "high in fibre", "source of protein", "reduced sugar". These have thresholds behind them.
- Comparative: "reduced" or "light" means less than the standard version of that product, not that the product is low in anything. Reduced-fat mayonnaise is still mostly fat.
- Loose: "natural", "farmhouse", "artisan", "wholesome", "inspired by". These mean approximately nothing.
- Health claims about a nutrient doing something in the body must be authorised, which is why the wording is often oddly stiff.
The traps worth knowing
"No added sugar" can still mean plenty of sugar from concentrated fruit juice. "Made with real fruit" tells you nothing about how much. "Multigrain" means several grains, refined or not; the word you want is wholegrain, ideally first in the list. "Plant-based" says nothing about salt, fat or processing.
Dates: use by is about safety and matters on meat, fish and dairy. Best before is about quality, and plenty of food is fine after it.
A thirty-second routine
Turn the packet over. Read the first three ingredients. Glance at the per-100g salt and sugar. Decide. That is it, and it is faster than reading the marketing on the front.
None of this makes a food good or bad on its own. A packet of biscuits is a packet of biscuits, and knowing exactly what is in it is not a reason to feel guilty about eating one. It is only a way of making sure the thing you bought is the thing you thought you were buying.