Problems in connection with the magnetic diagnostics applied in stellarators and its possibilities are discussed. The basic equations are obtained. These equations are used to connect the measurement results of the magnetic field outside the plasma with the plasma parameters and the magnetic configuration. The authors propose a method for determining the parameters of the magnetic configuration (the radius of the last magnetic surface, the rotational transform on this surface, the main harmonic amplitude of the stellarator field, etc.) that cannot be measured with sufficient precision by any existing diagnostics