It gets complicated

 Phenotypic plasticity refers to an organisms ability to change phenotype in response to their environment. The change in phenotype can be behavioral, morphological, or physiological; however, I will be discussing morphological changes in phenotype. 

One way to test for phenotypic plasticity is to compare the morphological characteristics of species across different environments. In the case of the side-blotched lizards, lizards living near lava flow exhibit a significantly darker color than those living in sand. This initial comparison of phenotypes led to the belief that these lizards exhibit phenotypic plasticity. 

After observing different phenotypes across different environments, phenotypic plasticity can further be confirmed by altering the environments of the lizards and observing any phenotypical changes. The side-blotched lizards exhibit a change in phenotype- from dark to light or light to dark, when exposed to the opposite environment. 

Both of these tests are able to show that organisms exhibit phenotypic plasticity. Plasticity can further be confirmed by looking at genes that affect the phenotype. For side-blotched lizards, darker lizards exhibit larger divergence of the PREP and PRKAR1a genes which are responsible for melanin production. The lizards living near the lava experience the greater divergence and have a darker complexion than lizards living on the sand.