In a fuel system of an internal combustion engine, the fuel quantity injected into a combustion chamber is a function of an activation period, during which a fuel injection device is activated. A pressure sensor detects the pressure in the fuel rail and provides a signal. The pressure in the fuel rail may be regulated to a setpoint pressure using a setting unit and employing the signal provided by the pressure sensor. The following method is provided for testing the fuel system: (a) establishing a test fuel quantity to be injected; (b) operating the internal combustion engine using a first setpoint pressure and a first activation period corresponding thereto and to the test fuel quantity to be injected; (c) detecting a speed- or torque-dependent variable characterizing the operating state; (d) operating the internal combustion engine using a second setpoint pressure and a second activation period corresponding thereto and to the test fuel quantity to be injected; (e) detecting a speed- or torque-dependent variable characterizing the operating state; (f) implementing an action if the variables detected in (c) and (e) differ by more than a limit value.