For years Gary McGowan, Nadia Macjaz and I studied window glass while we
worked for Louis Berger & Associates.  We found that it is very difficult to
distinguish between crown and cylinder glass by the naked eye.  There are
certain general trends such as cylinder glass tends to be thicker than crown
and lighter in color over time.  However, the only way that we found to
distinguish between the two was at the microscopic level.  From studies that
we did on samples of known window glass type we found that crown glass
contains microscopic air twists while cylinder glass does not.
 
Marie-Lorraine Pipes