I get this question a lot
Virtually all of the slab is covered by earth and floor coverings, so how do you inspect what you cannot see? There are a couple of points that need to be understood:
An experienced mechanic can tell you a lot about the performance of a car without looking at the engine. By listening to the engine as he drives the car putting it through its paces, he can learn a lot about the car. Did it start right up or did it barely turn over? If the latter, it might be a bad or weak battery. Perhaps the alternator is bad. Maybe the battery is weak because a cabin light was left on overnight. All this and more could be checked out without looking at the engine.
An experienced Professional Engineer can walk through a house and easily discover irregularities that are likely due to foundation distortion. Expansive soils react in predictable ways to the construction and presence of a slab-on-ground foundation. The house structure reacts in predictable ways to foundation distortion and the finish materials react to the distortion in the frame structure. An experienced Professional Structural engineer who specializes in foundation evaluations can tell a lot about the performance of a slab foundation by simply walking around and through a house.