A radio frequency (RF) probe (10) for a nuclear magnetic resonance check weighing system produces a uniform magnetic field at the center of RF probe (10), has minimal sensitivity to electrical interference from external sources, shapes the magnetic field to minimize cross coupling from packages such as drug vials (22) not under test, and presents minimal airflow obstruction. RF probe (10) includes a coil (100) with a plurality of conductive loops (102, 104) having a rectangular cross section carried inside two opposing, rectangular cross section housings (106, 108), respectively. Housings (106, 108) are spaced apart and in parallel, creating an "open probe" configuration permitting a conveyor belt (28) carrying vials (22) whose contents are to be checked to pass between housings (106, 108), and allows airflow surrounding the vials (22) and conveyor belt (28) to pass substantially without obstruction. Conductive loops (102, 104) and the housings (106, 108) carrying them may be placed such that the longitudinal axis of coil (100) is oriented in either horizontal or vertical spatial relation with conveyor belt (28).