Recently, electromagnetic waves possessing orbital angular momentum (OAM) have been widely studied recently in the optical and microwave regimes. In this article, we present a method for identifying OAM modes in the millimeter-wave regime, where direct phase measurements are not feasible in high-power environments. Instead, we rely on intensity measurements for OAM mode discrimination. Inspired by the fact that OAM beams exhibit asymmetric phase patterns, we used a method for OAM mode discrimination based on the interference pattern after passing through a double slit. Up to our knowledge, this article presents the first gyrotron-based OAM mode identification with the introduction of a method that uses Young's double slit interference. The proposed OAM mode discrimination method can be effectively applied to realize millimeter-wave wireless communication using OAM beams.