  This paper provides a novel proof for the sufficiency of certain well-known
criteria that guarantee the martingale property of a continuous, nonnegative
local martingale. More precisely, it is shown that generalizations of Novikov's
condition and Kazamaki's criterion follow directly from the existence of
F\"ollmer's measure. This approach allows to extend well-known criteria of
martingality from strictly positive to only nonnegative, continuous local
martingales.
