  We study the interacting, symmetrically coupled single impurity Anderson
model. By employing the nonequilibrium Green's function formalism, we establish
an exact relationship between the steady-state charge current flowing through
the impurity (dot) and its occupation. We argue that the steady state
current-occupation relation can be used to assess the consistency of simulation
techniques, and identify spurious transport phenomena. We test this relation in
two different model variants: First, we study the Anderson-Holstein model in
the strong electron-vibration coupling limit using the polaronic quantum master
equation method. We find that the current-occupation relation is violated
numerically in standard calculations, with simulations bringing up incorrect
transport effects. Using a numerical procedure, we resolve the problem
efficiently. Second, we simulate the Anderson model with electron-electron
interaction on the dot using a deterministic numerically-exact time-evolution
scheme. Here, we observe that the current-occupation relation is satisfied in
the steady-state limit---even before results converge to the exact limit.
