Plasma measurements in the Earth's magnetotail have revealed occasional existence of multiple plasma components containing a fast flowing beam embedded within a stationary population during dynamic episodes. We present here a quantitative procedure to extract plasma parameters at the source location of a fast flowing beam from the observed 3D velocity distribution. These parameters include the magnetic field at the source location of the beam as well as total electric potential drop experienced by the beam. This procedure is based on the Liouville's Theorem and the nonlinear least square Levenberg-Marquardt fitting method, under the assumptions of gyrotropic distribution and constant phase space density for a given magnetic moment in the ensemble. We demonstrate this technique with an event observed by Geotail 3D plasma measurements. The result shows that the fast-flowing beam population originated generally from a location with a magnetic field strength much less than that measured locally, had similar thermal properties as the local stationary population, and had undergone substantial electric field acceleration before detection at the observing site.