I hope nobody has mentioned it (this time around, ie!!), but could
not many fakes (esp fill-ins and such) be detected by using a very
bright (say, 400W Halon) light, holding the shell in front of it
(wearing dark glasses), and looking for dark areas that would represent
non-shell material?  One can even get flashlights that have this
powerful a beam, for situations where an outlet is not available!  This
approach could also spot many paint-jobs, by showing up irregularities
in the paint layer, and glue would also be exposed.
 
-Ross M.