We obtain a simple direct derivation of the differential equation governing
the entropy flow probability distribution function of a stochastic system first
obtained by Lebowitz and Spohn. Its solution agrees well with the experimental
results of Tietz et al [2006 {\it Phys. Rev. Lett.} {\bf 97} 050602]. A
trajectory-sampling algorithm allowing to evaluate the entropy flow
distribution function is introduced and discussed. This algorithm turns out to
be effective at finite times and in the case of time-dependent transition
rates, and is successfully applied to an asymmetric simple exclusion process