I had a driveway poured three years ago. After a crack in the walkway appeared and two more developed in the driveway itself, my husband began to get suspicious that I wasn't given a 4" slab. If we dig around the side of the drive and measure the concrete, should it be 4"? or could it settle to a little over 3".
If you are only interested in thickness of your concrete slab there is a method to measure it. Sometimes when someone doesn't know the strength of some old concrete element which was poured by someone else people hire specialist for this; they use special tool which is cylinder shaped and which drills the concrete and extracts concrete cylinder, which than you can measure. Don’t be afraid that you will damage the slab because it is only inch or two wide. On the other hand if you are wondering what is wrong and how to fix it you need to provide us with more details. People who wrote comments have asked the right questions, especially Ecnerwal.
Cracking concrete rarely has anything to do with its thickness.
If the concrete has no footing below the frost line, cracking is virtually certain in an area with hard freezes.
If a no-load paving like a concrete sidewalk cracks, it is either because of a root, or because it is improperly drained. If concrete sits in water, it will absorb the water and then crack when the water freezes. Sometimes a sidewalk will crack due to adiabatic expansion of an improper subsurface (like dirt), but 95% of the time it is a water-related problem.