But DavidReneke has more details here.
Finding tiny bits of space debris isn’t easy. Project Stardust collected and filtered through 300 kilograms (660 pounds) of material from a total collection area covering 30,000 square meters. Of these, about 500 rocks passed stringent scrutiny.
To pick out these tiny needles from the metaphorical haystack, scientists first sifted through the collected debris with magnets, since most ordinary chondrite-type meteorites have a high iron content. Next, the scientists washed the remainder and then painstakingly sorted the rocks by size and shape.
Finally, the final suspects are examined under a binocular microscope, where researchers looked for the luster and spherical shape indicative of ablation during atmospheric entry. Of the 500 particles collected, 48 were then embedded in resin and polished for further characterization. The micrometeorites collected are tiny, most just 300 to 400 micron in size. The largest of them are just under half a millimeter across, barely visible to the naked eye.
what discourages some people is that air pollution can mess up the results:
Howeer, later studies found that the abundance of magnetic microspherules dropped sharply away from urban areas, and modern pollution is full of metallic particulates that add a steady stream of false-positive “micrometeor wrongs,” confounding searchefforts.
and some experts who know what to look for search for them in desert areas, or in the ice of the polar regions.