  We show that residence time measure can be used to identify the geometrical
and transmission properties of a defect along a path. The model we study is
based on a one--dimensional simple random walk. The sites of the lattice are
regular, i.e., the jumping probabilities are the same in each site, except for
a site, called \emph{defect}, where the jumping probabilities are different. At
each side of the lattice an absorbing site is present. We show that by
measuring the fraction of particles crossing the channel and/or the typical
time they need to cross it, it is possible to identify the main features of the
lattice and of the defect site, namely, the jumping probabilities at regular
and at the defect sites and the position of the defect in the lattice.
