  We introduce an experimentally accessible method to measure a unique degree
of nonclassicality, based on the quantum superposition principle, for arbitrary
quantum states. We formulate witnesses and test a given state for any
particular value of this measure. The construction of optimal tests is
presented as well as the general numerical implementation. We apply this
approach on examples such as squeezed states, and we show how to formulate
conditions to certify a particular degree of nonclassicality for single- and
multimode radiation fields.
