Read the label.

Some great rules of thumb for reading food labels in the SMH:

.… skip the ‘Per serving’ column of the nutrition panel and reading the figures in the ‘Per 100g’ column instead.
….learn some ‘benchmark’ numbers that tell you whether yoghurt has too much sugar or if there’s not enough fibre in your muesli – there should be
at least 5g of fibre per 100g in a breakfast cereal, she says and
less than 10g of sugar per 100g of yoghurt.
….traditional oats have around 9g of fibre per 100g.
…. With any breakfast cereal, muesli included, it’s best to choose products with
no more than 20g of sugar per 100g,
but if a cereal or muesli contains fruit then up to 25g per 100g is acceptable, she says.
Too much salt in breakfast food is often a sign of a cheaper corn or wheat-based cereal.
“Salt adds flavour – without it some cereals would probably taste like cardboard,” she says. “Cereal should have no more than 400 mg sodium per 100g.”

Plus this handy guide: