Dundurn's right, bone / ivory and plastics / resins often fox even the experts.
One good trick is to get a magnifying glass and look underneath handles, in-carved bits etc. If this is moulded ie plastic or resin, you will see either a 'seam' of raised material or evidence of such a seam being filed away. If it is carved bone / ivory you will see the tool marks of the carver, extending the artwork under the handle.
With even greater magnification you can see the structure of the bone / ivory which difers from the structure of plasics but this is one mother of a magnification.