We show that the method of complete optical field reconstruction introduced by Debeau et al. [Opt. Lett.23, 1784 (1998)10.1364/OL.23.001784] allows a precise determination of the linewidth enhancement factor (LEF) of semiconductor lasers. We determine experimentally the LEF with 3% uncertainty. The method is rather simple and reliable, and is of interest in the context of microwave photonics. While particularly adapted to actively modulated devices, the proposed approach can in principle be applied to any laser exhibiting a time-periodic behavior.