A croissant is flour, butter, milk, yeast and three days of patience. You can tell most of what you need to know before tasting it.
Colour
It should be deep gold to chestnut, not pale. Colour is flavour: a pale croissant is an underbaked one.
Layers
Look at the cut end. You should see distinct steps, like the pages of a book. Ours have twenty-seven.
The inside
Tear it open. A good croissant has a honeycomb of open cells. If it is bready and dense, the butter melted into the dough before baking.
If it does not leave flakes on the table, it was not laminated with butter.
Sound and smell
Press the crust and it should crackle. It should smell of butter and nothing else.
Eat it within six hours of baking, and if you must keep it, warm it for three minutes in a hot oven. Never a microwave.