A method for verifying a control system (2) of a vessel (4), in which said control system (2) in its operative state receives sensor signals (7) from sensors (8) and command signals (9) from command input devices (10), and as a response provides control signals (13) to actuators (3) in order to maintain a desired position, velocity, course or other state of said vessel (4), characterized by the following steps: during a time (t0), disconnecting the reception of real sensor signals (7a, 7b, 7c, ..) and replacing said real sensor signals by a test sequence (T0) of artificial measurements (7a', 7b', 7c', ..) from a test signal source (41); letting said control system (2) work based on the artificial sensor signals (7, 7') to generate control signals (13') to be recorded as a response (S0) to said first test sequence (T0) for said first time (t0) on a control signal logger (42) and storing response (SO) to the test sequence (T0) as the control system's (2) ''signature'' response (S0); said method having the purpose of, at a later time (t1, t2, t3, ..), to use the test sequence (T0) input to the control system (2), and record a later response (S1, S2, S3, ...) and determining whether said later response similar to the signature response (S0) to verify that said control system (2) is unchanged, or not.