There is disclosed a method of generating an alert relating to a hydraulic actuation system for actuating a hydraulic cylinder, the hydraulic control system comprising a hydraulic fluid tank and a hydraulic fluid filter. The method comprises monitoring the pressure drop over the hydraulic fluid filter; determining a rate of change characteristic relating to the rate of change of pressure drop over at least one time period in which there is a hydraulic fluid flow through the hydraulic fluid filter; determining whether the hydraulic fluid is dirty based on at least the rate of change characteristic; and generating an alert if it is determined that the hydraulic fluid is dirty.