This letter describes a method to sense slight off‐stoichiometry (less than 2%–3%) in the cation ratio [Cu/(Pr+Ce) or Cu/(La+Sr)] of atomic beam fluxes by detecting precipitated impurity phases on the surface using reflection high‐energy electron diffraction during growth ofT′‐(Pr,Ce)2CuO4andT‐(La,Sr)2CuO4films. The method is based on the facts that off‐stoichiometry of fluxes inevitably produces precipitates of certain impurity phases on the surface in film growth of these oxides, and that the species of the impurity phases are solely determined by the type of off‐stoichiometry; copper rich or lanthanoid rich. Furthermore, it has been found that it is possible to recover a precipitate‐free surface, from a surface with precipitates produced by lanthanoid‐rich deposition, by later flux readjustment, but it is difficult from a surface with precipitates produced by copper‐rich deposition. ©1995 American Institute of Physics.