By tradition, jurists, statesmen, and scholars have looked exclusively to two factors to divine whether an emergent rule has attained customary international law status: 1) widespread State practice and 2) manifestations of a conviction that the practice is required by international law.
Scharf, Michael P.
"Accelerated Formation Of Customary International Law,"
ILSA Journal of International & Comparative Law: Vol. 20
, Article 5.
Available at: http://nsuworks.nova.edu/ilsajournal/vol20/iss2/5