The electronic states in a disordered solid may be localized or delocalized, depending on their energy and the disorder potential. By examining exciton dynamics with transient four-wave mixing experiments we show that low-energy excitons in CdTe/(Cd,Mg)Te quantum wells are localized, while higher-energy excitons are not. In the delocalized regime the excitonic dephasing rate decreases with increasing energy, a theoretically predicted, but experimentally unreported result.