An apparatus for identifying failed or suspect RF transmitting devices while the devices are in normal operation transmitting over-the-air RF signals, each of the RF transmitting devices including an identifier and being mobile, portable or stationary, includes an RF signal sensor receiving the RF signals and responsive thereto, an analyzer, and a GUI. The analyzer measuring signal parameters and determines operating characteristics of a device therefrom, determining the identifier, if each RF signal conforms to packet structure specifications, and bit-field information of each RF signal to discern therefrom a transmission type, measures RF frequency offset, correlates the bit-field information with the RF frequency offset utilizing the identifier; and repeats the last three steps to determine a range of RF frequency offset deviations over time. The GUI displays the range in a form used to identify an RF frequency offset range of the RF transmitting device.