The front of a bottle is marketing. The back is a legal document. Learning to read the back takes about ten minutes and permanently changes what you are willing to pay for.
The one rule that does most of the work
Ingredients are listed in descending order of concentration until you reach 1%. Below that threshold they can be listed in any order. So the first five ingredients are the product. Everything after "phenoxyethanol" or a similar preservative is usually present in fractions of a percent.
This is why a serum can legally advertise a hero ingredient that sits second-to-last on the list, below the fragrance.
Five things worth looking for
1. A stated percentage
If a brand is confident in a concentration, they print it. "15% L-ascorbic acid" is a claim you can hold them to. "Contains vitamin C" is not.
2. Actives in the top five
Niacinamide, glycerin, ceramides, squalane and hyaluronic acid all work at concentrations that put them near the top of a list when they are doing real work.
3. A short list overall
Not because long lists are bad in principle, but because a fifty-ingredient formula for a single-purpose product usually means a lot of texture and scent engineering and not much else.
4. A preservative
Anything water-based needs one. A "preservative-free" water formula is either using a preservative under a friendlier name, or it is a microbiology problem waiting to happen.
5. Packaging that matches the ingredient
Vitamin C and retinol degrade with light and air. Selling either in a clear jar with a screw lid tells you something about how seriously the formulation was taken.
Three things worth skipping
Denatured alcohol high on the list
Written as alcohol denat. or SD alcohol. It makes formulas feel light and dry fast, and in quantity it is hard on a compromised barrier. Fine near the bottom; a problem in the top five.
Fragrance in a product you use on damaged skin
Fragrance is not inherently harmful, and plenty of people use it happily for years. But it is one of the most common contact irritants, and it adds no functional benefit to a barrier cream. If your skin is unhappy, it is the cheapest variable to remove.
"Natural" as a substitute for information
Essential oils are natural and are among the more common irritants in cosmetics. Ask what an ingredient does, not where it came from.
Thirty-second version: read the first five ingredients, look for a printed percentage, and check that the packaging protects what is inside.
What a list cannot tell you
Formulation quality, pH, particle size, delivery system and stability all matter enormously and none of them appear on an INCI list. Two products with near-identical lists can perform very differently. Reading the list rules out the obviously overpriced; it does not guarantee the rest.
General cosmetic information, not medical advice. Patch-test anything new on the inner forearm for 48 hours if you have known allergies.