The spiderweb, not a single line
Crazing does not look like a normal crack. Instead of one line running across the slab, you are looking at a web of very fine, interconnected lines covering a patch of the surface, sometimes the whole slab, in a pattern that looks a little like a cracked eggshell or a dry lake bed seen up close.
It usually shows up easiest to see when the surface is damp, right after a rain or a wash down, or when the light hits it at a low angle. On a dry day in flat light, the same slab can look almost smooth.
Why the top skin dries faster than the rest
Concrete cures through a chemical reaction between cement and water, not by drying out the way paint does, and that reaction happens through the full depth of the slab at once. The very top skin, though, is exposed directly to sun, wind and open air, and it can lose its surface moisture faster than the concrete just beneath it.
That top layer shrinks slightly as it loses moisture, and because it is bonded to the slightly less dry concrete right underneath, the same kind of tension that opens a joint crack plays out here in miniature, across a whole skin instead of along one planned line. The result is a network of hairline cracks that only go a fraction of an inch deep.
Where it tends to show up first
A finish that is troweled smooth and dense, rather than broom finished, tends to show crazing more readily, because a tighter, paste rich surface has less texture to hide the fine lines and can dry a little faster on top than a coarser finish does. Sun and wind on pour day push it further, the same conditions that make the first two days after any pour matter more than people expect.
A slab that was kept damp and shaded through its early cure, or one poured on a mild, calm day, is less likely to show much crazing at all, though nothing eliminates the possibility completely.
Why crazing is not a warning sign
Crazing lives entirely in the surface skin. It does not run down through the slab, it does not affect how much weight the concrete can carry, and it does not mean the mix or the pour was done wrong. Plenty of sound, long lasting driveways and patios have some crazing somewhere on them.
It is different from dusting, where the surface itself turns powdery underfoot, and different from a real structural crack, where you can feel a step or watch it travel with the seasons. Crazing is texture, not damage.