  We analyze the quantum process tomography (QPT) in the presence of
decoherence, focusing on distinguishing local and non-local decoherence
mechanisms for a two-partite system from experimental QPT data. In particular,
we consider the $\sqrt{\rm iSWAP}$ gate realized with superconducting phase
qubits and calculate the QPT matrix $\chi$ in the presence of several local and
non-local decoherence processes. We determine specific patterns of these
decoherence processes, which can be used for a fast identification of the main
decoherence mechanisms from an experimental $\chi$-matrix.
