We give a new characterization of biinfinite Sturmian sequences in terms of indistinguishable asymptotic pairs. Two asymptotic sequences on a full $mathbb{Z}$-shift are indistinguishable if the sets of occurrences of every pattern in each sequence co
incide up to a finitely supported permutation. This characterization can be seen as an extension to biinfinite sequences of Pirillos theorem which characterizes Christoffel words. Furthermore, we provide a full characterization of indistinguishable asymptotic pairs on arbitrary alphabets using substitutions and biinfinite characteristic Sturmian sequences. The proof is based on the well-known notion of derived sequences.
