An apparatus and method for estimating blockage within a liquid strainer used in a marine vessel is provided. The apparatus includes a sensor for measuring the rate of liquid flow through the strainer and a sensor for measuring the operational speed of a device utilizing the strained liquid. The apparatus also includes a control panel and central processing unit configured to receive at least one measurement from the flow sensor and receive at least one measurement from the speed sensor. The central processing unit then compares the relationship of the measured liquid flow rate and measured device speed to a predetermined clean strainer reference relationship of liquid flow rate and device speed at an equivalent device speed to determine the degree of strainer blockage. Visible and audible indicators are actuated when blockage becomes excessive.