Scientists determine if a mass extinction occurred primarily by comparing the number of genera over time. This approach involves analyzing the diversity of life forms across various geological time periods and looking for significant drops in the number of species (or genera) present in the fossil record, which can indicate a mass extinction event.
While counting the number of fossils and examining rock layers can provide useful information about environmental conditions and biodiversity, the most direct evidence of mass extinction is typically derived from shifts in genera diversity and the patterns found in the fossil record. Changes in soil acidity or rock color may reflect environmental changes but are not primary indicators of mass extinction events themselves.