The resolution of the plasma potential measurements by a 30° electrostatic energy analyzer of a heavy-ion beam probe (HIBP) is strongly related to the accuracy of knowledge of the incident angle of the beam ( ). Initially it is determined by the calculations of the beam trajectories in the confining magnetic field of the plasma device. However, because the magnetic-field simulation codes are not exact, an uncertainty in always exists. A common practice to reduce the influence of this uncertainty consists in tuning the analyzer geometry parameters in such a way as to minimize the dependence on the variation of that results from a scan of the beam across the plasma. Still, the attachment of the energy analyzer to the diagnostic port cannot be controlled with enough precision and, therefore, an unknown shift out from the chosen optimal range of is predictable. This article proposes a method for the direct evaluation of the incident angles during operation of HIBP in plasma experiments. It is based on the addition of a second split-plate detector along a 30° exit axis of the analyzer, and on the comparison of the respective normalized beam currents. The experimental verification of the method on a test facility has shown that an accuracy better than =0.1° can be attained.