Ed,

Hold your cone up with the aperture facing you and the spire upwards --
pretty much the way it is displayed in every shell book.  If the aperture is
on your right (as it will be 99.999% of the time) then it is a normal,
right-handed, dextral shell. If the opening in on your left, it is
left-handed, or sinistral -- very rare in cones and cowries.  And yes both
can be sinistral.  If you hold a cowrie up in the same manner you will see
it also has the main body of the shell on one side and the opening on the
other.

Hope this helps,

Tom