Sadly, they aren't really, except after the fact by looking at filters.
Particulate matter is removed from the air by HEPA filters located in the Common Cabin Air Assemblies (a fan/heat exchanger device). When my information was current, the filters were cleaned approximately every 90 days.
Some problems caused by accumulation of particles/dust are listed in the old paper ISS ECLSS Technology Evolution for Exploration
The unanticipated accumulation of a thick layer of lint on an
intermodule ventilation fan's inlet screen led to degraded flow
on-orbit. The buildup was due to passage of small lint through the
subsystems inlet filter screens and subsequent aggregation into larger
clumps which caused the flow blockage. As a result of the lack of fine
particle settling in microgravity both the size distribution and quantity
of particulate load differ substantially from that upon which the
ventilation system's inlet screen was designed. To resolve the problem
on ISS, a duct lint filter will be installed.
Ambient dust has caused
the Node I smoke detector to trigger false positive alarms. Although
the detector is located behind the cabin fan's inlet screen, dust is
accumulating necessitating periodic cleaning by the crew.
Characterization and accounting for background dust levels must be
accounted for, or an alternate, dust- insensitive detector is needed.
The gasses making up the cabin atmosphere are monitored by a mass spectrometer called the Major Constituent Analyzer, but there is nothing to monitor particulate matter.
Sources: None public except the linked paper, because NASA doesn't care to publish technical details about the ISS in an organized way.
Addendum: User uhoh noted this experiment which at the time of writing had obtained one HEPA filter for post-flight analysis.
