A suitable etching technique can distinguish the appearance of microscopic features such as grain boundaries (GBs), annealing twins (TBs), and precipitates, unlike non-metallic inclusions, voids, and microcracks, which are visible in as-polished conditions. The difficulties arise when a semi-automatic or automatic grain size (GS) analyzer detects the TBs in addition to GBs concurrently. This study introduces a novel etching technique that has selectively highlighted the GBs without or faintly emphasizing TBs in heat-treated additively manufactured (AM) and wrought Nickel alloy 718, respectively. The authors then measured the GS using the comparison and intercept procedure (Lineal and Abrams three-circle) according to ASTM E112-2013. Moreover, the comparative study identified the comparison procedure as the most effective approach based on accuracy, post-processing, and time consumption. However, it had limitations in terms of precision and applicability. In contrast, the intercept procedure emerged as the preferred choice for higher accuracy demands.