How do I know if my slab has voids or if it just needs leveling?

A slab that is sunken typically has both settlement and voids and the settlement is the visible symptom and the void is the cause. A slab that is still at grade but flexes underfoot, sounds hollow when tapped, or has cracked without visible settlement likely has a void without having yet settled. Scaldino's assessment distinguishes between the two conditions and determines whether the repair is filling only, lifting only, or both.