I took the 3 pieces I purchased from eBay to Tiffany’s for cleaning and all 3 pieces were cleaned. I have been told they will not clean if they are not authentic Tiffany. 2 out the 4 pieces I purchased also came with a legitimate receipt. That helps with wondering whether they are authentic.
Just be careful. If it's a decent fake and the SA misses it, they can and will clean it. It actually not that uncommon. The only way to truly tell is to send it off for a full buff and polish and the jewellers will let you know quickly.