Try checking for the logos instead.
Do all raybans have rb etching. On fakes the etching will either be missing or of poor quality. If it's not glass then don't panic, some newer designs use other materials; In some glasses, the hinges are attached with philip screws to tighten these.
If the etching is missing, or a low quality or painted on the glasses will be. Furthermore the logo should be etched onto the lens. If the etching is missing, or a low quality or painted on the glasses will be fake.