A method for testing a line trap of the type whose blocking characteristics are a function of its resonant frequencies without requiring the line trap to be de-energized along with the transmission line to which the trap is connected by injecting a signal into the transmission line over a range of frequencies including at least one of the line trap's expected resonant frequencies; measuring the magnetic field strength produced by the current through the line trap's main coil and the total current flowing through the line trap, and computing the ratio of the magnetic fields where the ratio is computed over a range of frequencies that includes the expected resonant frequency(s) of the line trap, such that the resonant frequencies are indicated where said ratio is at a local maxima or minima.