Currently there is no definitive test or product parameter available to conclude that the MODIS aerosol product is measuring dust or smoke.  Over ocean a low fine mode fraction is a strong indicator of dust but is not definitive.  Often ancillary information must be included to try to make this determination.  This ancillary information can include: AERONET data, AIRS dust product (over ocean), MISR data, OMI aerosol index, VIIRS suspended matter product, back trajectories which link to a dust or smoke origin.