Fourier domain mode locked (FDML) laser are fast swept light sources. Measuring the linewidth and coherence length of such light sources is not straightforward, but very important for a physical understanding of FDML lasers and their performance in optical coherence tomography (OCT). In order to characterize the dynamic (“instantaneous”) linewidth, we performed beat signal measurements between a stationary narrowband continuous wave laser and an FDML laser and detected the signals with a 63 GHz real time oscilloscope. The evaluation of the beat signals of consecutive FDML wavelength sweeps yields information about the phase evolution within one sweep and over several sweeps. These measurements suggest the existence of a distinct comb like mode structure of the FDML laser and help to determine the locking strength of individual modes (comb lines).