  In this paper we study how to determine if a linear biochemical network
satisfies the detailed balance condition, without knowing the details of all
the reactions taking place in the network. To this end, we use the formalism of
response functions $R_{ij} (t) $ that measure how the system reacts to the
injection of the substance $j$ at time $t=0$, by measuring the concentration of
the substance $i \neq j$ for $t >0$. In particular, we obtain a condition
involving two reciprocal measurements (i.e.~$R_{ij}(t), \, R_{ji}(t)$) that is
necessary, but not sufficient for the detailed balance condition to hold in the
network. Moreover, we prove that this necessary condition is also sufficient if
a topological condition is satisfied, as well as a stability property that
guarantees that the chemical rates are not fine-tuned.
