The temporal profile of relativistic laser-plasma-accelerated electron bunches has been characterized. Coherent transition radiation at THz frequencies, emitted at the plasma-vacuum boundary, was measured through electro-optic sampling. Frequencies up to the crystal detection limit of 4 THz were observed. Comparison between data and theory indicates that THz radiation from bunches with structure shorter than ≃50  fs (root-mean-square) is emitted. The measurement demonstrates both shot-to-shot stability of the laser-plasma accelerator and femtosecond synchronization between bunch and probe beam.