A non-perturbing probe for drift velocities in high-density plasma flows is described. Coherent Thomson scattering has been used for direct and local measurements. Two pulsed laser beams probe the plasma flow alternately from two different directions yielding measurable Doppler shifts. The high repetition frequency of the laser system permits the detection of small relative Doppler shifts of the scattered light pulses by means of a correlation technique. The influence of fluctuations of the scattered light intensity is eliminated by employing two detection units with different spectral characteristics. Local values of drift velocities and electron densities can be measured simultaneously at particular points in the plasma flow. Drift velocities between 0.3 and 2.7 km s^-1 and electron densities between 10²² and 10²³ m^-3, depending on the discharge conditions of the plasma thruster (current, gas composition), are found. The laser Doppler technique presented provides a spatial resolution better than 1 mm³.