It is of great interest to determine the deuteron temperature or density in tokamak fusion plasmas from total neutron rate measurements. This can be done easily during ohmic heating or H° injection, where the velocity distribution function of the plasma deuterons is Maxwellian. However, during 0° injection the distribution function is highly non-Maxwellian and until now deuteron temperatures or denstities could only be evaluated with the aid of complex codes. Here we present a fast interpretation code using a Fokker-Planck formalism and identify the parameter ranges where deuteron temperatures or densities can be determined from neutron rate measurements during neutral beam heating. As an example of its application, ion temperatures and densities are evaluated for JET discharges.