The most practical way to judge chocolate quality is to check the cocoa percentage on the label and read the ingredient list. High-cocoa dark chocolate usually contains little beyond cocoa butter, cocoa mass and sugar; as the percentage drops, milk powder, vegetable fat and flavourings start showing up on the list.
If it's going into a recipe for baking, chocolate labelled "couverture" is the better choice, since its higher cocoa butter content lets it melt smoothly and set with a glossy finish. For chocolate meant to be eaten as-is, the cocoa percentage comes down to personal taste; some brands even print "suitable for baking" on the wrapper to make that distinction explicit.
What's the difference between dark, milk and white chocolate?
Dark chocolate is cocoa mass and cocoa butter combined with sugar; it contains no milk powder and its cocoa content typically runs 50% to 85%. Milk chocolate has milk powder added, giving it a softer flavour, and usually sits in the 20-40% cocoa range.
White chocolate contains no cocoa mass at all — just cocoa butter, milk powder and sugar — which technically makes it the furthest thing from "chocolate" and leaves it without any cocoa flavour. All three melt at different points, so swapping one for another in a recipe isn't recommended.
What does the cocoa percentage on the label actually tell you?
A "70% cocoa" label means that share of the product's total weight is cocoa mass and cocoa butter; the rest is sugar and, sometimes, a small amount of an additive like lecithin. The higher the percentage, the more intense and less sweet the chocolate tastes.
If the ingredient list mentions "vegetable fat" or "cocoa butter substitute", part of the cocoa butter has been replaced with another fat; these products are cheaper but don't melt or set as glossily as real chocolate.
Which chocolate should you choose for baking?
For ganache, mousse and coatings, couverture chocolate is the right pick because its cocoa butter content is higher than standard chocolate, giving it a runnier melt and a glossier set once cooled. Standard baking chocolate can be melted too, but it's more likely to dull in a coating.
If it's just going to be chopped into cake or cookie dough, couverture isn't necessary; what matters there is that the chocolate mostly holds its shape in the batter, which is why chocolate chips are made with less fat than couverture.
Other things worth checking when buying chocolate
A whitish film on the surface (bloom) doesn't mean the chocolate has gone bad; it's usually cocoa butter rising to the surface after a temperature swing, and it melts back fine, though it's best avoided for coating work where appearance matters. Chocolate that snaps with a clean "crack" is well-tempered and fresh.
Finally, check the best-before date and storage conditions; chocolate that hasn't been kept cool and dry loses both flavour and texture over time. An undamaged, tightly sealed package is also a sign the quality has been preserved; a torn or open wrapper should be avoided, since chocolate picks up surrounding smells easily.