Determining movement of a Radio Frequency Identification (RFID) tag first establishes a phase difference/frequency model comprising a set of phase-wrapped local rates-of-change and an intercept point. RFID tag readings are then made to measure phase differences between the interrogation signal and the tag response at different frequencies. A correlation is determined between the measured phase differences versus frequency and the phase-wrapped local phase rates-of-change of the phase difference/frequency model. If the correlation is better than or equal to a predetermined limit, the tag is stationary, or if the correlation is worse than the predetermined limit, the tag is moving.