In needle-based dispensing systems, the target amount of liquid to be dispensed is in many cases very well controlled. However, the amount of liquid that is actually reaching the receiving well may show a discrepancy, because part of the liquid leaving the inner needle space remains on the outer surface of the dispense needle, and so a small amount of liquid is “lost” in the dispensing act. A sensor arrangement according to the present invention is suitable for detecting the presence of any liquid from the aspiration reservoir that is left behind on the outside of the dispense needle, before the dispense step is actually started. The “lost” volume can also be measured very accurately by inserting the tip of a dispense needle into a hollow metallic cylinder, forming a capacitor, whereby the capacitance value depends on the volume of the “lost” dispense volume.