I have a time series representing the result of a complex calculation (physical simulation). Due to round-off errors and approximation errors, there will be some "noise" on the data series. In some cases, the physical simulation represents a "steady state" situation meaning that, in theory, the time series should be a flat line. However, if the steady state is not well calculated there will be a drift in the time series.
What would be the best algorithm to check whether there is drift or not? Some smoothing filter followed by a kind of linear function approximation $ax+b$ and check for a small $a$ (define small?)