Perhaps in speaking of "fake" shells we should differentiate between
out-and-out manufactured specimens (a Cypraea leucodon or Cypraea
valentia created by reworking a Cypraea mappa or Cypraea tigris) and
a "retouched" shell, which actually is the species it is represented
to be, but has had its pattern and gloss restored.  Either type of
fake can be identified as fake by a dip in a non water-soluble
organic solvent like xylene, toluene, chloroform, etc.  Paint thinner
will also work.  These solvents quickly attack the paints and resins
used in such restorations, but won't hurt an actual shell.  Of
course, this approach is not advisable if you want to keep the fake
looking as it does!  These solvents will not just identify a fake,
they will also "ruin" it.  Some water-soluble solvents, particularly
acetone, will also expose fakes, but I shy away from using them on
shells.  They are extremely drying, and may have a deliterious effect
on some pigments which are naturally hydrated.
 
Paul M.