A system and method for determining whether the pressure in a fluid line upstream of an infusion pump is becoming lower as pumping draws fluid from the fluid line and fluid container due to a lack of proper venting. An initial pressure is sensed by an upstream pressure sensor at the start of an infusion. After a period of time, which may be determined either by monitoring infusion time or the number of pump cycles completed, a second pressure is sampled and compared to the initial pressure. Various analytical methods may be applied to the sensed pressures to determine upstream pressures or trends. Where an upstream pressure reading indicates a lack of proper venting, an alert signal is provided. Continuous monitoring may occur in which succeeding samples are compared to preceding samples to determine upstream pressures or trends.