I do know how to tell. Usually shell have chisel marks, if hand carved. The white might show the salmon color in spots, especially locks of hair. From your pictures it looks more like shell than resin or plastic. Pictures are not always great though. Look and just tell me if the things I mentioned are there. If it exist I will bid. The brooch is vintage, probably between mid 1940 and mid 1950.
It has a back on it so you can not see thru it. I don't see any chisel marks. But, I cannot see very well. Haven't had my glasses changed in over 2 years:( The pics I took are very much what the piece looks like, if this helps at all.
I meant more like shell, not resin or plastic. Again, it is not in my hand so I cannot tell you. If you can't tell go outside in sunlight and you can see best. Sometimes the pink or salmon is semi-translucent in spots.