It gets complicated

To understand what plasticity is and how to determine what makes a trait "plastic" is to start by defining the term. Plasticity is an organism's ability to change their phenotype as a response to an environmental condition change. With that established, there are many ways you can determine if a trait is plastic. First, you would have to uncover if there are any environmental conditions that would impact the organim's trait. Next, you could do a controlled experiment where you alter a specific condition to see if that changes the organism's traits or you could look at different species that are closely related or the same species in different environments to see if the trait is impacted by different environmental conditions. If that trait differs significantly between environments, then it would be considered plastic. This would then need to be compared to the genetic variation to determine that this is not due to genetic variation. If all of this leads to the conclusion that the trait is plastic, then there would be opportunity to see if the trait can revert back to the original phenotype in the original environment. In this example, there is a mountain, grassland, and tree environment that birds are required to adapt to. As you can see, the birds in the trees have two twos forward and two backward while the other two environments have toes 2 in front and 2 in back. This is due to birds having to feed on insects in the trees like woodpeckers. Their feet are adapted to hold onto trees allowing them to better hold onto bark. Not that this is a true example of plasticity.