Synchronous communications mode detection is disclosed for unlicensed spectrum communications. To detect synchronous mode communications, an observing node monitors on-off transitions of communication transmissions on a shared communication spectrum. The observing node detects the on-off transition spacing of the on-off transitions over a sliding window of a detection period duration. The node may determine the number of candidate synchronous on-off transition spacings of the detected on-off transition spacings that are equal to an integer multiple of a synchronization period. If the number of candidate synchronous on-off transition spacings exceeds a threshold value the observing node will declare detection of a synchronous operation mode of the shared communication spectrum.