Start at the joint line
Look at where the crack actually runs before anything else. A crack that follows a control joint, or one that stays close to it, is usually just shrinkage finding the path the joint was cut to give it. That is normal and it is not what people mean when they ask if a crack is structural.
A crack that wanders away from every joint and cuts across open concrete is a different animal. It is telling you the stress that caused it did not come from ordinary shrinkage, since shrinkage cracking follows the weak line the joint created. Walk the whole length of it before you decide, since a crack can start at a joint and then leave it partway across the slab.
Run a hand across it
Feel for a step. Set your fingers flat across the crack and see if one side sits higher than the other. Any difference you can feel, even a small one, means the ground on one side of that crack is behaving differently than the ground on the other side, which is settlement, not shrinkage.
A crack with both sides still level, even a long one, is still in cosmetic territory. Level is the word that matters here more than length. Check it in a couple of spots along the run too, since a crack can start level near one end and pick up a step further along.
Measure what a fingernail and a pencil tell you
Width is the next check. A hairline crack, one you can barely catch with a fingernail, sits at one end of the range and rarely means anything beyond normal shrinkage. A crack wide enough to drop a pencil into sits at the other end, and width like that on its own is worth a second look even before you check for a step.
None of these three checks stand alone particularly well. A wide crack with a level surface and no growth over a season can still be cosmetic. It is the combination that tells the real story, not any single measurement by itself.
- Hairline, barely catches a fingernail: usually shrinkage
- A gap you can fit a coin edge into: worth a closer look
- Wide enough for a pencil to drop in: worth a call on its own
Watch it across a season, not a week
A crack that shows up and then stops is one thing. A crack that keeps opening wider, or that grows a step it did not have a month ago, is telling you the movement underneath it has not finished. Mark the ends of it if you want an easy way to track that, since a crack that keeps growing past where it started is the clearest sign something is still moving.
What it usually means when all three line up
A crack that runs away from a joint, has a step you can feel, and keeps widening is pointing at the ground underneath rather than the concrete itself. That is a base question, not a cosmetic one, and it is what our tear out and replacement page walks through in full once you get to that point.