  We study an analog quantum simulator coupled to a reservoir with a known
spectral density. The reservoir perturbs the quantum simulation by causing
decoherence. The simulator is used to measure an operator average, which cannot
be calculated using any classical means. Since we cannot predict the result, it
is difficult to estimate the effect of the environment. Especially, it is
difficult to resolve whether the perturbation is small or if the actual result
of the simulation is in fact very different from the ideal system we intend to
study. Here, we show that in specific systems a measurement of additional
correlators can be used to verify the reliability of the quantum simulation.
The procedure only requires additional measurements on the quantum simulator
itself. We demonstrate the method theoretically in the case of a single spin
connected to a bosonic environment.
