In this paper, a practical system for detecting (identifying the on-off state of) the load applied to a construction manipulator (front load) by using a hydraulic sensor is presented. Such an industrial application requires high accuracy and robustness considering the uncertainty in hydraulic pressure-based force measurement. Therefore, our system is composed of (a) identifying the dominant error force component and binarizing the analog external cylinder force, (b) evaluating detection conditions to handle indeterminate states and redefining three-valued outputs (on, off, or not determinate (ND)), and (c) outputting the reliability-ensured front load decision by combining all the cylinder decisions through priority analysis. Experiments were conducted using an instrumented hydraulic arm. Results indicate that our system properly detects on, off, and ND states of the front load in various detection conditions.