We perform the two - point diagnostic for the Om ( z ) function proposed by Sahni { \it etal } in 2014 for the Starobinsky and Hu \& Sawicki models in f ( R ) gravity .
We show that the observed values of the Omh ^ { 2 } function can be explained in f ( R ) models while in LCDM the Omh ^ { 2 } funticon is expected to be a redshift independent number .
We perform the analysis for some particular values of \Omega _ { m } ^ { 0 } founding a cumulative probability ( P ( \chi ^ { 2 } \leq \chi ^ { 2 } _ { { \it model } } ) ) P \sim 0.16 or \sim 0.09 for the better cases versus a cumulative probability of P \sim 0.98 in the \Lambda CDM scenario .
We also show that these models present a characteristic signature around the interval between z \sim 2 and z \sim 4 , that could be confronted with future observations using the same test .