1. A method for determining a response to administration of a cancer chemotherapeutic agent to an individual, comprising: (a) collecting a first tissue or cell sample from the individual before exposing the individual to the cancer chemotherapeutic agent; (b) collecting a second tissue or cell sample from the individual after exposing the individual to the cancer chemotherapeutic agent; (c) staining the first and second tissue or cell samples with one or a multiplicity of stains to determine senescence-associated β-Gal (SA β-Gal) activity, p21 expression, or both SA β-Gal activity and p21 expression; (d) measuring the optical density of the stained tissue or cell samples in step (c), wherein the stained tissue or cell samples are illuminated with light having a wavelength absorbed by the one or a multiplicity of stains; determining whether SA β-Gal activity, expression of p21, or both SA β-Gal activity and expression of p21 was increased following exposure to the cancer chemotherapeutic agent.