The gas discharged from the airbag as it is being inflated escapes through the vent holes in the stitching seams in the front of the airbag.  If clothing from the occupant comes into contact with the airbag when it is at or near maximum inflation, the hot gas can cause singe patterns on the front of the clothing, characteristic of the seam patterns.  These singe marks appear as small dots or smears.  In addition, contact between a person and/or their clothing with an airbag can leave trace evidence behind on the airbag and/or the person/clothing.  Differences in the design of the airbags can lead to different class characteristics of singe patterns and therefore lead to the inclusion or exclusion of certain airbags as the source.  Wearing a seatbelt may reduce the opportunity for clothing to become singed.