We present DropleX, a system that enables liquid sensing using the capacitive touchscreen of commodity tablets. DropleX detects microliter-scale liquid samples and performs non-invasive, through-container measurements to detect whether a drink has been spiked or whether a sealed liquid has been contaminated. These capabilities are enabled by a physics-informed mechanism that disables the touchscreen's built-in adaptive filters, originally designed to reject the effects of liquid drops such as rain, without requiring any hardware modifications. We model the touchscreen's sensing capabilities, limits, and non-idealities to inform a signal processing and learning-based sensing pipeline. Across experiments, the system achieves high accuracy for microliter-scale adulteration detection, trace chemical concentration thresholding, and through-container adulterant detection.