Final answer:
The spiral-arm structure of the Milky Way Galaxy has been most effectively measured and evaluated by radio observations, especially through the discovery of 21-cm radio wave emissions from cool hydrogen, which are not obscured by interstellar dust.
Step-by-step explanation:
The spiral-arm structure of the Milky Way Galaxy has been evaluated using various observational techniques. The most effective of these has been radio observations, which reveal details about the gas within the galaxy's disk. The discovery of the 21-cm radio wave emissions from cool hydrogen was pivotal, as these emissions pass through interstellar dust, which obscures other wavelengths.
This allowed astronomers to map the galaxy's structure despite the dust that makes direct optical observations at large distances difficult. The infrared observations have also been utilized to gain insight into the distribution of stars within the Milky Way, providing a complementary view that is not affected by dust. Combining observations at these wavelengths has been integral to understanding the spiral arms that emerge from the bar structure at the galaxy's center, along with several fainter arms and shorter spurs where our own Sun is located.