The topological charge (ℓ) is the parameter that defines the amount of orbital angular momentum (OAM) of beams carrying optical vortices. In this article we expand on a robust experimental method to determine the topological charge that we previously developed [B. Khajavi & E. J.Galvez, Opt. Lett. 42, 1516-1519 (2017)]. This method consist of using a pair of confocal lenses and a wedged optical flat as a shearing interferometer. The interference pattern produced by a pure OAM mode directly reveals the magnitude and sign of the topological charge: two conjoined forks with |ℓ| + 1 number of tines pointing away from (towards) one another for ℓ > 0 (ℓ < 0). Superpositions of two modes with the same value of |ℓ| but with the opposite sign produce shearing interference patterns that can be used the infer the value of ℓ and the relative weight of the component modes.