How might you determine whether a trait is a result of genetic or environmental factors?

By doing an experiment using genetically identical organisms and controlled variables. For example multiple clones of a plant could be propagated and then each clone grown in an environment that is different by one variable to the others, as well as one clone that would be considered the base or control. By observing whether the plants exhibits different traits in different environments you could then determine if the trait is a result of environmental or genetic factors.