<p>The global drive toward carbon–neutral marine propulsion has positioned ammonia as a compelling alternative fuel due to its zero-carbon emissions. However, ammonia flash-boils easily, and there is no consensus in the literature on which optical techniques to use. Hence, this study aims to identify suitable optical methods to characterize flashing liquid ammonia sprays. Ammonia sprays were investigated in a constant volume chamber under ambient pressures (0.2–20 bar), corresponding to pressure ratio (R_P) values from 42.85 to 0.43 at a fuel temperature of 20 °C. Four optical diagnostics: Diffused Back Illumination (DBI), Mie scattering, shadowgraphy, and schlieren, were applied to compare their suitability. A single-hole injector and high-speed imaging were used to measure spray tip penetration (STP) and spray width (SW). All methods produced comparable STP values; however, differences were observed under flash boiling. DBI captured the liquid core, while shadowgraphy and schlieren detected a wider spray, indicating that they detected ammonia vapor and dispersed fine droplets; however, schlieren's additional detection of shock waves at high pressures made data processing more difficult. Mie scattering lost accuracy in wide plumes due to scattering-induced signal loss. It was concluded that a combination of DBI and shadowgraphy is most suitable to detect the dense liquid core, the vapor, and any dispersed atomized droplets.</p>