If the particle density is high enough, seeded flows can be used similarly to dyed flows. In water, you might as well just use a dye, but in a gas flow there are few substances that will behave as a dye, so particle concentrations that are high enough to appear uniform are used for boundary methods.
When particle concentrations are low enough that individual particles can be imaged, other flow visualization techniques become available: particle pathlines, aka particle tracks, and even a quantitative method – particle image velocimetry (PIV).