  Recently B. Derrida [Phys. Rev. E 55, 3705 (1997)] introduced a numerical
technique that allows one to measure the fraction of persistent spins in a
coarsening nonequilibrium system at finite temperature. In the present work we
extend this method in a way that domain walls can be clearly identified. To
this end we consider three replicas instead of two. As an application we
measure the surface area of coarsening domains in the two-dimensional Ising
model at finite temperature. We also discuss the question to what extent the
results depend on the algorithmic implementation.
