  We show that the scale dependence of the fluctuations of the natural time
itself under time reversal provides a useful tool for the discrimination of
seismic electric signals (critical dynamics) from noises emitted from man made
sources as well as for the determination of the scaling exponent. We present
recent data of electric signals detected at the Earth's surface, which confirm
that the value of the entropy in natural time as well as its value under time
reversal are smaller than that of the entropy of a "uniform" distribution.
