Hi James,
 
To detect either of these finishes, you can try alcohol for shellac, and
acetone for lacquer. Pick an inconspicuous spot and test a small area. If the
shell is coated, the finish will soften. You might also want to try and scrape
the surface at a 90° angle with a blade that is not very sharp...the finish
will flake, or shave off (and you'll feel the blade "dig in" a little...don't
use an extremely sharp blade, or you may actually be shaving off bits of the
shell itself).  A less damaging method is to just eyeball the shells looking
for ripples, pits or bumps, and uneven areas. The latter technique may not
give conclusive results on its own, since cowries can exhibit all of those
"defects" naturally. A practiced eye can tell the difference, however.
 
Congrats on your "Eugie"! Keep an eye to the ground, you're bound to find some
things of interest to you.
 
Ken Zentzis
Wichita, Kansas