We present a straightforward theoretical approach for the direct observation of the orbital angular momentum (OAM) charge inherent in structured light. Our method involves utilizing phase-shift interference of beams to discern the sign and magnitude of the topological charge associated with Laguerre–Gaussian (LG) beams. Specifically, we demonstrate that the topological charge of an LG beam can be ascertained through the interference with a corresponding helical wavefront beam possessing an opposite charge due to the reflection in Mach–Zehnder-like interferometer. The determination of the topological charge magnitude is facilitated by examining the interference pattern’s half number of petals. Additionally, our study reveals that the petal-like interference pattern undergoes azimuthal shift, with the direction of rotation being contingent upon the sign of the topological charge of the beam. Notably, for a negative phase shift, a anti-clockwise azimuthal shift corresponds to a positive topological charge, while an clockwise rotation is associated with a negative topological charge. This work contributes to the advancement of methods for characterizing structured light based on OAM charge, with potential applications in diverse fields such as optics, telecommunications and topological photonics.