The measurement of neutral particle fluxes generated on doping beams in a plasma at different velocities gives real-time information on the local ion temperature and the relative hydrogenic composition of the plasma. The plasma must consist of more than one species (H, D, T) and each species must have a Maxwellian distribution with the same temperature. No other knowledge of plasma or beam parameters is required. The method seems to be well suited to all heating schemes in a plasma that do not preferentially heat one species, for example Ohmic heating or fusion heating. Effects of the beam halo, scattering or perturbation by the probing beam are discussed qualitatively